High energy density redox flow device

ABSTRACT

Redox flow devices are described in which at least one of the positive electrode or negative electrode-active materials is a semi-solid or is a condensed ion-storing electroactive material, and in which at least one of the electrode-active materials is transported to and from an assembly at which the electrochemical reaction occurs, producing electrical energy. The electronic conductivity of the semi-solid is increased by the addition of conductive particles to suspensions and/or via the surface modification of the solid in semi-solids (e.g., by coating the solid with a more electron conductive coating material to increase the power of the device). High energy density and high power redox flow devices are disclosed. The redox flow devices described herein can also include one or more inventive design features. In addition, inventive chemistries for use in redox flow devices are also described.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/970,753, entitled “High Energy Density Redox Flow Device,” filed Dec.16, 2010. U.S. patent application Ser. No. 12/970,753 is acontinuation-in-part of U.S. patent application Ser. No. 12/484,113,entitled “High Energy Density Redox Flow Device,” filed Jun. 12, 2009,which claims priority under 35 U.S.C. §119(e) to U.S. Provisional PatentApplication Ser. No. 61/060,972, entitled “High Energy Density RedoxFlow Battery,” filed Jun. 12, 2008 and U.S. Provisional PatentApplication Ser. No. 61/175,741, filed May 5, 2009, entitled “HighEnergy Density Redox Flow Battery.” U.S. patent application Ser. No.12/970,753 also claims priority under 35 U.S.C. §119(e) to U.S.Provisional Patent Application Ser. No. 61/287,180, entitled “HighEnergy Density Redox Flow Device,” filed Dec. 16, 2009. Each of theseapplications is incorporated herein by reference in its entirety for allpurposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Grant NumberDE-FC26-05NT42403 awarded by the Department of Energy. The governmenthas certain rights in this invention.

INCORPORATION BY REFERENCE

All patents, patent applications and documents cited herein are herebyincorporated by reference in their entirety for all purposes.

BACKGROUND

A battery stores electrochemical energy by separating an ion source andan ion sink at differing ion electrochemical potential. A difference inelectrochemical potential produces a voltage difference between thepositive and negative electrodes; this voltage difference will producean electric current if the electrodes are connected by a conductiveelement. In a battery, the negative electrode and positive electrode areconnected by two conductive elements in parallel. The external elementconducts electrons only, and the internal element (electrolyte) conductsions only. Because a charge imbalance cannot be sustained between thenegative electrode and positive electrode, these two flow streams supplyions and electrons at the same rate. In operation, the electroniccurrent can be used to drive an external device. A rechargeable batterycan be recharged by application of an opposing voltage difference thatdrives electronic current and ionic current in an opposite direction asthat of a discharging battery in service. Thus, the active materials ofrechargeable batteries need to be able to accept and provide ions.Increased electrochemical potentials produce larger voltage differencesthe cathode and anode, and increased voltage differences increase theelectrochemically stored energy per unit mass of the device. Forhigh-power devices, the ionic sources and sinks are connected to theseparator by an element with large ionic conductivity, and to thecurrent collectors with high electronic conductivity elements.

Rechargeable batteries can be constructed using static negativeelectrode/electrolyte and positive electrode/electrolyte media. In thiscase, non-energy storing elements of the device comprise a fixed volumeor mass fraction of the device; thereby decreasing the device's energyand power density. The rate at which current can be extracted is alsolimited by the distance over which cations can be conducted. Thus, powerrequirements of static cells constrain the total capacity by limitingdevice length scales.

Redox flow batteries, also known as a flow cells or redox batteries orreversible fuel cells are energy storage devices in which the positiveand negative electrode reactants are soluble metal ions in liquidsolution that are oxidized or reduced during the operation of the cell.Using two reversible redox couples, liquid state redox reactions arecarried out at the positive and negative electrodes. A redox flow celltypically has a power-generating assembly comprising at least anionically transporting membrane separating the positive and negativeelectrode reactants (also called catholyte and anolyte respectively),and positive and negative current collectors (also called electrodes)which facilitate the transfer of electrons to the external circuit butdo not participate in the redox reaction (i.e., the current collectormaterials themselves do not undergo Faradaic activity). Redox flowbatteries have been discussed by C. Ponce de Leon, A. Frias-Ferrer, J.Gonzalez-Garcia, D. A. Szantos and F. C. Walsh, “Redox Flow Batteriesfor Energy Conversion,” J. Power Sources, 160, 716 (2006), M.Bartolozzi, “Development of Redox Flow Batteries: A HistoricalBibliography,” J. Power Sources, 27, 219 (1989), and by M.Skyllas-Kazacos and F. Grossmith, “Efficient Vanadium Redox Flow Cell,”Journal of the Electrochemical Society, 134, 2950 (1987).

Differences in terminology for the components of a flow battery andthose of conventional primary or secondary batteries are herein noted.The electrode-active solutions in a flow battery are typically referredto as electrolytes, and specifically as the catholyte and anolyte, incontrast to the practice in lithium ion batteries where the electrolyteis solely the ion transport medium and does not undergo Faradaicactivity. In a flow battery, the non-electrochemically active componentsat which the redox reactions take place and electrons are transported toor from the external circuit are known as electrodes, whereas in aconventional primary or secondary battery they are known as currentcollectors.

While redox flow batteries have many attractive features, including thefact that they can be built to almost any value of total charge capacityby increasing the size of the catholyte and anolyte reservoirs, one oftheir limitations is that their energy density, being in large partdetermined by the solubility of the metal ion redox couples in liquidsolvents, is relatively low. Methods of increasing the energy density byincreasing the solubility of the ions are known, and typically involveincreasing the acidity of the electrode solutions. However, suchmeasures which may be detrimental to other aspects of the celloperation, such as by increasing corrosion of cell components, storagevessels, and associated plumbing. Furthermore, the extent to which metalion solubilities may be increased is limited.

In the field of aqueous electrolyte batteries, and specificallybatteries that utilize zinc as an electroactive material, electrolytesthat comprise a suspension of metal particles and in which thesuspension is flowed past the membrane and current collector, have beendescribed. See for example U.S. Pat. Nos. 4,126,733 and 5,368,952 andEuropean Patent EP 0330290B1. The stated purpose of such electrodes isto prevent detrimental Zn metal dendrite formation, to preventdetrimental passivation of the electrodes, or to increase the amount ofzincate that can be dissolved in the positive electrode as the celldischarges. However, the energy density of such fluidized bed batterieseven when electrolytes with a suspension of particles are used remainsrelatively low.

Thus, there remains a need for high energy-density and highpower-density energy storage devices.

SUMMARY

Redox flow energy storage devices are described in which at least one ofthe positive electrode or negative electrode-active materials mayinclude a semi-solid or a condensed ion-storing liquid reactant, and inwhich at least one of the electrode-active materials may be transportedto and from an assembly at which the electrochemical reaction occurs,producing electrical energy. By “semi-solid” it is meant that thematerial is a mixture of liquid and solid phases, for example, such as aslurry, particle suspension, colloidal suspension, emulsion, gel, ormicelle. “Condensed ion-storing liquid” or “condensed liquid” means thatthe liquid is not merely a solvent as it is in the case of an aqueousflow cell catholyte or anolyte, but rather, that the liquid is itselfredox-active. Of course, such a liquid form may also be diluted by ormixed with another, non-redox-active liquid that is a diluent orsolvent, including mixing with such a diluent to form a lower-meltingliquid phase, emulsion, or micelles including the ion-storing liquid.

In one aspect, a redox flow energy storage device is described. Theredox flow energy storage device includes:

-   -   a positive electrode current collector, a negative electrode        current collector, and an ion-permeable membrane separating the        positive and negative current collectors;    -   a positive electrode disposed between the positive electrode        current collector and the ion-permeable membrane; the positive        electrode current collector and the ion-permeable membrane        defining a positive electroactive zone accommodating the        positive electrode;    -   a negative electrode disposed between the negative electrode        current collector and the ion-permeable membrane; the negative        electrode current collector and the ion-permeable membrane        defining a negative electroactive zone accommodating the        negative electrode;    -   where at least one of the positive and negative electrode        includes a flowable semi-solid or condensed liquid ion-storing        redox composition which is capable of taking up or releasing the        ions during operation of the cell.

In some embodiments, both of the positive and negative electrodes of theredox flow energy storage device include the flowable semi-solid orcondensed liquid ion-storing redox compositions.

In some embodiments, one of the positive and negative electrodes of theredox flow energy storage device includes the flowable semi-solid orcondensed liquid ion-storing redox composition, and the remainingelectrode is a conventional stationary electrode.

In some embodiments, the flowable semi-solid or condensed liquidion-storing redox composition includes a gel.

In some embodiments, the steady state shear viscosity of the flowablesemi-solid or condensed liquid ion-storing redox composition of theredox flow energy storage device is between about 1 cP and about1,500,000 cP or between about 1 cP and 1,000,000 cP at the temperatureof operation of the redox flow energy storage device.

In some embodiments, the ion is selected from the group consisting ofLi⁺, Na⁺, H⁺.

In some embodiments, the ion is selected from the group consisting ofLi⁺ and Na⁺, Mg²⁺, Al³⁺, and Ca²⁺.

In some embodiments, the flowable semi-solid ion-storing redoxcomposition includes a solid including an ion storage compound.

In some embodiments, the ion is a proton or hydroxyl ion and the ionstorage compound includes those used in a nickel-cadmium or nickel metalhydride battery.

In some embodiments, the ion storage compound stores ions by undergoinga displacement reaction or a conversion reaction.

In some embodiments, the ion is lithium and the ion storage compound isselected from the group consisting of metal fluorides such as CuF₂,FeF₂, FeF₃, BiF₃, CoF₂, and NiF₂.

In some embodiments, the ion is lithium and the ion storage compound isselected from the group consisting of metal oxides such as CoO, Co₃O₄,NiO, CuO, and MnO.

In some embodiments, the ion storage compound comprises an intercalationcompound.

In some embodiments, the ion is lithium and the ion storage compoundcomprises an intercalation compound.

In some embodiments, the ion is sodium and the ion storage compoundcomprises an intercalation compound.

In some embodiments, the ion is lithium and the ion storage compoundincludes an intercalation compound selected from compounds with theformula Li_(1-x-z)M_(1-z)PO₄, wherein M includes at least one first rowtransition metal selected from the group consisting of Ti, V, Cr, Mn,Fe, Co and Ni, wherein x is from 0 to 1 and z can be positive ornegative.

In some embodiments, the ion is lithium and the ion storage compoundincludes an intercalation compound selected from compounds with theformula (Li_(1-x)Z_(x))MPO₄, where M is one or more of V, Cr, Mn, Fe,Co, and Ni, and Z is a non-alkali metal dopant such as one or more ofTi, Zr, Nb, Al, or Mg, and x ranges from 0.005 to 0.05.

In some embodiments, the ion is lithium and the ion storage compoundincludes an intercalation compound selected from compounds with theformula LiMPO₄, where M is one or more of V, Cr, Mn, Fe, Co, and Ni, inwhich the compound is optionally doped at the Li, M or O-sites.

In some embodiments, the ion is lithium and the ion storage compoundincludes an intercalation compound selected from the group consisting ofA_(x)(M′_(1-a)M″_(a))_(y)(XD₄)_(z), A_(x)(M′_(1-a)M″_(a))_(y)(DXD₄)_(z),and A_(x)(M′_(1-a)M″_(a))_(y)(X₂D₇)_(z), wherein x, plus y(1-a) times aformal valence or valences of M′, plus ya times a formal valence orvalence of M″, is equal to z times a formal valence of the XD₄, X₂D₇, orDXD₄ group; and A is at least one of an alkali metal and hydrogen, M′ isa first-row transition metal, X is at least one of phosphorus, sulfur,arsenic, molybdenum, and tungsten, M″ is any of a Group IIA, IIIA, IVA,VA, VIA, VIIA, VIIIA, IB, IIB, IIIB, IVB, VB, and VIB metal, and D is atleast one of oxygen, nitrogen, carbon, or a halogen.

In some embodiments, the ion is lithium and the ion storage compoundincludes an intercalation compound selected from the group consisting of(A_(1-a)M″_(a))_(x)M′_(y)(XD₄)_(z), (A_(1-a)M″_(a))_(x)M′_(y)(DXD₄)z and(A_(1-a)M″_(a))_(x)M′_(y)(X₂D₇)_(z), where (1-a)x plus the quantity axtimes the formal valence or valences of M″ plus y times the formalvalence or valences of M′ is equal to z times the formal valence of theXD₄, X₂D₇ or DXD₄ group, and A is at least one of an alkali metal andhydrogen, M′ is a first-row transition metal, X is at least one ofphosphorus, sulfur, arsenic, molybdenum, and tungsten, M″ any of a GroupIIA, IIIA, IVA, VA, VIA, VIIA, VIIIA, IB, IIB, IIIB, IVB, VB, and VIBmetal, and D is at least one of oxygen, nitrogen, carbon, or a halogen.

In some embodiments, the ion is lithium and the ion storage compoundincludes an intercalation compound selected from the group consisting ofordered rocksalt compounds LiMO₂ including those having the α-NaFeO₂ andorthorhombic-LiMnO₂ structure type or their derivatives of differentcrystal symmetry, atomic ordering, or partial substitution for themetals or oxygen, where M includes at least one first-row transitionmetal but may include non-transition metals including but not limited toAl, Ca, Mg, or Zr.

In some embodiments, the flowable semi-solid ion-storing redoxcomposition includes a solid including amorphous carbon, disorderedcarbon, graphitic carbon, or a metal-coated or metal-decorated carbon.

In some embodiments, the flowable semi-solid ion-storing redoxcomposition includes a solid including a metal or metal alloy ormetalloid or metalloid alloy or silicon.

In some embodiments, the flowable semi-solid ion-storing redoxcomposition includes a solid including nanostructures includingnanoparticles, nanowires, nanorods, nanotripods, and nanotetrapods.

In some embodiments, the flowable semi-solid ion-storing redoxcomposition includes a solid including an organic redox compound.

In some embodiments, the positive electrode includes a flowablesemi-solid ion-storing redox composition including a solid selected fromthe group consisting of ordered rocksalt compounds LiMO₂ including thosehaving the α-NaFeO₂ and orthorhombic-LiMnO₂ structure type or theirderivatives of different crystal symmetry, atomic ordering, or partialsubstitution for the metals or oxygen, wherein M includes at least onefirst-row transition metal but may include non-transition metalsincluding but not limited to Al, Ca, Mg, or Zr and the negativeelectrode includes a flowable semi-solid ion-storing redox compositionincluding a solid selected from the group consisting of amorphouscarbon, disordered carbon, graphitic carbon, or a metal-coated ormetal-decorated carbon.

In some embodiments, the positive electrode includes a flowablesemi-solid ion-storing redox composition including a solid selected fromthe group consisting of A_(x)(M′_(1-a)M″_(a))_(y)(XD₄)_(z),A_(x)(M′_(1-a)M″_(a))_(y)(DXD₄)_(z), andA_(x)(M′_(1-a)M″_(a))_(y)(X₂D₇)_(z), and where x, plus y(1-a) times aformal valence or valences of M′, plus ya times a formal valence orvalence of M″, is equal to z times a formal valence of the XD₄, X₂D₇, orDXD₄ group, and A is at least one of an alkali metal and hydrogen, M′ isa first-row transition metal, X is at least one of phosphorus, sulfur,arsenic, molybdenum, and tungsten, M″ any of a Group IIA, IIIA, IVA, VA,VIA, VIIA, VIIIA, IB, IIB, IIIB, IVB, VB, and VIB metal, D is at leastone of oxygen, nitrogen, carbon, or a halogen and the negative electrodeincludes a flowable semi-solid ion-storing redox composition including asolid selected from the group consisting of amorphous carbon, disorderedcarbon, graphitic carbon, or a metal-coated or metal-decorated carbon.

In some embodiments, the positive electrode includes a flowablesemi-solid ion-storing redox composition including a compound with aspinel structure.

In some embodiments, the positive electrode includes a flowablesemi-solid ion-storing redox composition including a compound selectedfrom the group consisting of LiMn₂O₄ and its derivatives; layered-spinelnanocomposites in which the structure includes nanoscopic regions havingordered rocksalt and spinel ordering; so-called “high voltage spinels”with a potential vs. Li/Li⁺ that exceeds 4.3V including but not limitedto LiNi_(0.5)Mn_(1.5)O₄; olivines LiMPO₄ and their derivatives, in whichM includes one or more of Mn, Fe, Co, or Ni, partially fluorinatedcompounds such as LiVPO₄F, other “polyanion” compounds as describedbelow, and vanadium oxides V_(x)O_(y) including V₂O₅ and V₆O₁₁.

In some embodiments, the negative electrode includes a flowablesemi-solid ion-storing redox composition including graphite, graphiticboron-carbon alloys, hard or disordered carbon, lithium titanate spinel,or a solid metal or metal alloy or metalloid or metalloid alloy thatreacts with lithium to form intermetallic compounds, including themetals Sn, Bi, Zn, Ag, and Al, and the metalloids Si and Ge.

In some embodiments, the redox flow energy storage device furtherincludes a storage tank for storing the flowable semi-solid or condensedliquid ion-storing redox composition, and the storage tank is in flowcommunication with the redox flow energy storage device.

In some embodiments, the redox flow energy storage device includes aninlet for introduction of the flowable semi-solid or condensed liquidion-storing redox composition into the positive/negative electroactivezone and an outlet for the exit of the flowable semi-solid or condensedliquid ion-storing redox composition out of the positive/negativeelectroactive zone. In some specific embodiments, the redox flow energystorage device further includes a fluid transport device to enable theflow communication. In certain specific embodiments, the fluid transportdevice is a pump. In certain specific embodiments, the pump is aperistaltic pump.

In some embodiments, the flowable semi-solid or condensed liquidion-storing redox composition further includes one or more additives. Incertain specific embodiments, the additives includes a conductiveadditive. In certain other embodiments, the additive includes athickener. In yet other specific embodiments, the additive includes acompound that getters water.

In some embodiments, the flowable semi-solid ion-storing redoxcomposition includes an ion-storing solid coated with a conductivecoating material. In certain specific embodiments, the conductivecoating material has higher electron conductivity than the solid. Incertain specific embodiments, the solid is graphite and the conductivecoating material is a metal, metal carbide, metal nitride, or carbon. Incertain specific embodiments, the metal is copper.

In some embodiments, the redox flow energy storage device furtherincludes one or more reference electrodes.

In some embodiments, the flowable semi-solid or condensed liquidion-storing redox composition of the redox flow energy storage deviceprovides a specific energy of more than about 150 Wh/kg at a totalenergy of less than about 50 kWh.

In some embodiments, the semi-solid or condensed-liquid ion-storingmaterial of the redox flow energy storage device provides a specificenergy of more than about 200 Wh/kg at total energy less than about 100kWh, or more than about 250 Wh/kg at total energy less than about 300kWh.

In some embodiments, the condensed-liquid ion-storing material includesa liquid metal or metal alloy.

In some embodiments, the flowable redox composition is electricallyconductive. In some embodiments the flowable redox composition has anelectrical conductivity of at least about 10⁻⁶ S/cm, at least about 10⁻⁵S/cm, at least about 10⁻⁴ S/cm, or at least about 10⁻³ S/cm at atemperature at which the energy storage device is operated. In someembodiments, the semi-solid ion-storing redox composition iselectrically conductive in its flowing and/or non-flowing state. In someembodiments, said composition has an electrical conductivity of at leastabout 10⁻⁶ S/cm, at least about 10⁻⁵ S/cm, at least about 10⁻⁴ S/cm, orat least about 10⁻³ S/cm at a temperature at which the energy storagedevice is operated.

In some embodiments, the ion-permeable membrane includespolyethyleneoxide (PEO) polymer sheets or Nafion™ membranes.

In some embodiments, a method of operating a redox flow energy storagedevice is described. The method includes:

providing a redox flow energy storage device including:

-   -   a positive electrode current collector, a negative electrode        current collector, and an ion-permeable membrane separating the        positive and negative current collectors;    -   a positive electrode disposed between the positive electrode        current collector and the ion-permeable membrane; the positive        electrode current collector and the ion-permeable membrane        defining a positive electroactive zone accommodating the        positive electrode;    -   a negative electrode disposed between the negative electrode        current collector and the ion-permeable membrane; the negative        electrode current collector and the ion-permeable membrane        defining a negative electroactive zone accommodating the        negative electrode;    -   where at least one of the positive and negative electrode        includes a flowable semi-solid or condensed liquid ion-storing        redox composition which is capable of taking up or releasing the        ions during operation of the cell;

transporting the flowable semi-solid or condensed liquid ion-storingredox composition into the electroactive zone during operation of thedevice.

In some embodiments, in the method of operating a redox flow energystorage device, at least a portion of the flowable semi-solid orcondensed liquid ion-storing redox composition in the electroactive zoneis replenished by introducing new semi-solid or condensed liquidion-storing redox composition into the electroactive zone duringoperation.

In some embodiments, the method of operating a redox flow energy storagedevice further includes:

transporting depleted semi-solid or condensed liquid ion-storingmaterial to a discharged composition storage receptacle for recycling orrecharging.

In some embodiments, the method of operating a redox flow energy storagedevice further includes:

applying an opposing voltage difference to the flowable redox energystorage device; and transporting charged semi-solid or condensed liquidion-storing redox composition out of the electroactive zone to a chargedcomposition storage receptacle during charging.

In some embodiments, the method of operating a redox flow energy storagedevice further includes:

applying an opposing voltage difference to the flowable redox energystorage device; and

transporting discharged semi-solid or condensed liquid ion-storing redoxcomposition into the electroactive zone to be charged.

As used herein, positive electrode and cathode are used interchangeably.As used herein, negative electrode and anode are used interchangeably.

The energy storage systems described herein can provide a high enoughspecific energy to permit, for example, extended driving range for anelectric vehicle, or provide a substantial improvement in specificenergy or energy density over conventional redox batteries forstationary energy storage, including for example applications in gridservices or storage of intermittent renewable energy sources such aswind and solar power.

In some embodiments, a redox flow energy storage device is provided. Theredox flow energy storage device can comprise a first, outer electrodecurrent collector, a second, inner electrode current collector disposedat least partially within the first electrode current collector, and anion-permeable medium at least partially separating said first and secondelectrode current collectors; a first electrode active material at leastpartially disposed between said first electrode current collector andsaid ion-permeable medium; and a second electrode active material atleast partially disposed between said second electrode current collectorand said ion-permeable medium; wherein at least one of said first andsecond electrode active materials comprises a fluid, and at least one ofthe first electrode current collector and the second electrode currentcollector is capable of being rotated about its longitudinal axisrelative to the other electrode current collector.

In one set of embodiments, a flowable ion-storing redox composition fora redox flow energy storage device is described. In some embodiments,the redox flow energy storage device comprises a positive electrodeactive material, a negative electrode active material, and anion-permeable medium separating said positive and negative electrodeactive materials, wherein at least one of said positive and negativeelectrode active materials comprises the flowable ion-storing redoxcomposition which is capable of taking up or releasing said ions duringoperation of the device, wherein said flowable ion-storing redoxcomposition comprises at least one compound selected from a ketone; adiketone; a triether; a compound containing 1 nitrogen and 1 oxygenatom; a compound containing 1 nitrogen and 2 oxygen atoms; a compoundcontaining 2 nitrogen atoms and 1 oxygen atom; a phosphorous containingcompound, and/or fluorinated, nitrile, and/or perfluorinated derivativesof these.

In some embodiments, a source of acoustic energy for a redox flow energystorage device is provided. In some embodiments, the redox flow energystorage device comprises a positive electrode active material, anegative electrode active material, and an ion-permeable mediumseparating said positive and negative electrode active materials,wherein at least one of said positive and negative electrode activematerials comprises a flowable ion-storing redox composition which iscapable of taking up or releasing said ions during operation of thedevice, wherein the flowable ion-storing redox composition comprises asolid, and said source of acoustic energy is constructed and arranged toinhibit the accumulation of the solid within the redox flow energystorage device and/or to reduce the viscosity of the flowableion-storing redox composition within the redox flow energy storagedevice.

In one set of embodiments, an in-line sensor for a redox flow energystorage device is described. In some embodiments, the redox flow energystorage device comprises a positive electrode active material, anegative electrode active material, an ion-permeable medium separatingsaid positive and negative electrode active materials, wherein at leastone of said positive and negative electrode active materials comprises aflowable ion-storing redox composition which is capable of taking up orreleasing said ions during operation of the device, and an in-linesensor constructed and arranged to determine a property of the flowableion-storing redox composition.

In some embodiments, a flowable ion-storing redox composition for aredox flow energy storage device. The redox flow energy storage devicecan comprise a positive electrode active material, a negative electrodeactive material, and an ion-permeable medium separating said positiveand negative electrode active materials, wherein at least one of saidpositive and negative electrode active materials comprises a flowableion-storing redox composition which is capable of taking up or releasingsaid ions during operation of the cell, wherein the flowable ion-storingredox composition comprises an aqueous liquid carrier, and the ioncomprises Li⁺ or Na⁺.

In some embodiments, a redox flow energy storage device comprises apositive electrode active material, a negative electrode activematerial, and an ion-permeable medium separating said positive andnegative electrode active materials, wherein at least one of saidpositive and negative electrode active materials comprises a flowableion-storing redox composition which is capable of taking up or releasingsaid ions during operation of the device; and a source of mixing fluidin fluid communication with and/or located within a volume in which theflowable ion-storing redox composition is disposed, wherein the mixingfluid is immiscible with the flowable ion-storing redox composition.

In one set of embodiments, a redox flow energy storage device comprisesa first electrode active material of a first polarity; a secondelectrode active material of a second, opposite polarity; anion-permeable medium separating the first and second electrode activematerials, wherein at least one of the first and second electrode activematerials comprises a flowable ion-storing redox composition which iscapable of taking up or releasing said ions during operation of thecell; and a movable surface in contact with the flowable ion-storingredox composition, wherein the movable surface is constructed andarranged to at least partially direct the flow of the flowableion-storing redox composition through the redox flow energy storagedevice.

In some embodiments, a flowable ion-storing redox composition for aredox flow energy storage device is described, wherein the redox flowenergy storage device comprises a positive electrode active material, anegative electrode active material, and an ion-permeable mediumseparating said positive and negative electrode active materials,wherein at least one of said positive and negative electrode activematerials comprises the flowable semi-solid or condensed liquidion-storing redox composition which is capable of taking up or releasingsaid ions during operation of the device, wherein said flowableion-storing redox composition comprises at least one of an ether, aketone, a diether, diketone, an ester, a triether, a carbonate; anamide, a sulfur containing compound; a phosphorous containing compound,an ionic liquid, and fluorinated, nitrile, and/or perfluorinatedderivatives of these.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter is described with reference to the drawings, whichare intended to be illustrative in nature and not intended to belimiting of the invention, the full scope of which is set forth in theclaims that follow.

FIG. 1A is a cross-sectional illustration of the redox flow batteryaccording to one or more embodiments.

FIG. 1B includes a cross-sectional illustration of a redox flow batteryaccording to one set of embodiments.

FIG. 1C includes a cross-sectional illustration of a redox flow batteryaccording to one set of embodiments.

FIG. 1D includes a schematic illustration of an energy storage devicecomprising a plurality of posts, according to one set of embodiments.

FIG. 1E includes a schematic illustration of a flow channel that can beused, in some embodiments, within an energy storage device.

FIGS. 1F-1G include exemplary schematic illustrations of an energystorage device comprising a plurality of augers.

FIG. 1H includes an exemplary schematic illustration of an energystorage device.

FIG. 1J includes a schematic illustration of an energy storage deviceinto which a mixing fluid is transported, according to one set ofembodiments.

FIG. 1K includes an exemplary schematic illustration of an energystorage device in which a mixing fluid such as a gas is generated.

FIG. 1L includes a schematic illustration of an energy storage devicecomprising a plurality of track drives, according to one set ofembodiments.

FIG. 1M includes an exemplary schematic illustration of an energystorage device comprising a plurality of rotatable axels.

FIG. 2 is a schematic illustration of an exemplary redox flow cell for alithium battery system.

FIG. 3 is a schematic illustration of an exemplary redox flow cell for anickel battery system.

FIG. 4 is a schematic illustration of an exemplary redox flow batteryusing reference electrodes to monitor and optimize cell performance.

FIG. 5 illustrates cycling performance of anode slurries with varyingcopper plating load.

FIG. 6 illustrates a representative plot of voltage as a function ofcharging capacity for the cathode slurry half-cell.

FIG. 7 illustrates a representative plot of voltage as a function ofcharging capacity for the anode slurry half-cell.

FIG. 8 illustrates a representative plot of voltage as a function oftime (lower panel) and the corresponding charge or discharge capacity(upper panel) for a electrochemical cell with cathode and anodeslurries.

FIG. 9 illustrates a representative plot of the cathode dischargecapacity vs. cycle number.

FIG. 10 illustrates the galvanostatic lithium insertion and extractioncurves for the suspension at a relatively high C/1.4 rate.

FIG. 11 includes a schematic illustration of an in-line sensor,according to one set of embodiments.

FIG. 12 includes, according to one set of embodiments, a plot of voltageas a function of time.

FIGS. 13A-13B include (A) an exemplary plot of viscosity versus shearrate for suspensions of nanoparticulate carbon (Ketjen black) and LiCoO₂(LCO) in alkyl carbonate electrolyte and (B) an exemplary Nyquist plotshowing AC impedance of alkyl carbonate electrolyte alone, andsuspensions of particles in electrolyte, according to one set ofembodiments.

FIG. 14 includes, according to some embodiments, a flow cellconfiguration for energy storage.

FIG. 15 includes exemplary plots of the state of charge, current, andvoltage as a function of time for a semi-solid half-flow-cell testinvolving a multi-step galvanostatic charge/discharge of a LiCoO₂suspension flowing continuously at 20.3 mL/min, separated fromstationary Li metal negative electrode by microporous separator film,according to one set of embodiments.

FIG. 16 includes, according to some embodiments, exemplary plots ofvoltage, charge storage capacity, and current as a function of time fora semi-solid anode suspension measured during continuous flow at 10mL/min vs. a lithium metal counterelectrode.

FIG. 17 includes an exemplary plot of voltage as a function of capacityfor a dual electrolyte lithium ion cell using a semi-solid nanoscaleolivine cathode and a semi-solid lithium titanate spinel anode,according to one set of embodiments.

FIG. 18 includes, according to one set of embodiments, an exemplary plotof voltage vs. capacity for a system comprising a MCMB graphite anodesemi-solid suspension tested in a dioxolane based electrolyte vs. alithium metal counterelectrode.

FIG. 19 includes an exemplary plot of voltage as a function of specificcapacity for a LiCoO₂ cathode semi-solid suspension tested against alithium metal counter-electrode in a [Li(G4)]TFSI ionic liquidelectrolyte at C/11 rate, according to one set of embodiments.

DETAILED DESCRIPTION

An exemplary redox flow energy storage device 100 is illustrated in FIG.1A. Redox flow energy storage device 100 may include a positiveelectrode current collector 110 and a negative electrode currentcollector 120, separated by an ion permeable separator 130. Currentcollectors 110, 120 may be in the form of a thin sheet and are spacedapart from separator 130. Positive electrode current collector 110 andion permeable separator 130 define an area, 115, herein after referredto as the “positive electroactive zone” that accommodates the positiveflowable electrode active material 140. Negative electrode currentcollector 120 and ion permeable separator 130 define an area, 125,herein after referred to as the “negative electroactive zone” thataccommodates the negative flowable electrode active material 150. Theelectrode-active materials can be flowable redox compositions and can betransported to and from the electroactive zone at which theelectrochemical reaction occurs. The flowable redox composition caninclude a semi-solid or a condensed liquid ion-storing electroactivematerial, and optionally a fluid for supporting or suspending the solidor condensed ion-storing liquid electrolyte. As used herein, semi-solidrefers to a mixture of liquid and solid phases, such as a slurry,particle suspension, colloidal suspension, emulsion, or micelle. In someembodiments, the emulsion or micelle in a semi-solid includes a solid inat least one of the liquid-containing phases. As used herein, condensedliquid or condensed ion-storing liquid refers to a liquid that is notmerely a solvent as it is in the case of an aqueous flow cell catholyteor anolyte, but rather that the liquid is itself redox-active. Theliquid form can also be diluted by or mixed with another,non-redox-active liquid that is a diluent or solvent, including mixingwith such a diluents to form a lower-melting liquid phase, emulsion ormicelles including the ion-storing liquid.

The positive electrode flowable material 140 can enter the positiveelectroactive zone 115 in the direction indicated by arrow 160. Positiveelectrode material 140 can flow through the electroactive zone and exitat the upper location of the electroactive zone in the directionindicated by arrow 165. Similarly, the negative electrode flowablematerial 150 can enter the negative electroactive zone 125 in thedirection indicated by arrow 170. Negative electrode material 150 canflow through the electroactive zone and exits at the upper location ofthe electroactive zone in the direction indicated by arrow 175. Thedirection of flow can be reversed, for example, when alternating betweencharging and discharging operations. It is noted that the illustrationof the direction of flow is arbitrary in the figure. Flow can becontinuous or intermittent. In some embodiments, the positive andnegative redox flow materials are stored in a storage zone or tank (notshown) prior to use. In some embodiments, the flowable redox electrodematerials can be continuously renewed and replaced from the storagezones, thus generating an energy storage system with very high energycapacity. In some embodiments, a transporting device is used tointroduce positive and negative ion-storing electroactive materials intothe positive and negative electroactive zones, respectively. In someembodiments, a transporting device is used to transport depletedpositive and negative ion-storing electroactive materials out of thepositive and negative electroactive zones, respectively, and intostorage tanks for depleted electroactive materials for recharging. Insome embodiments, the transporting device can be a pump or any otherconventional device for fluid transport. In some specific embodiments,the transporting device is a peristaltic pump.

During operation, the positive and negative electroactive materials canundergo reduction and oxidation. Ions 190 can move across ion permeablemembrane 130 and electrons can flow through an external circuit 180 togenerate current. In a typical flow battery, the redox-active ions orion complexes undergo oxidation or reduction when they are in closeproximity to or in contact with a current collector that typically doesnot itself undergo redox activity. Such a current collector may be madeof carbon or nonreactive metal, for example. Thus, the reaction rate ofthe redox active species can be determined by the rate with which thespecies are brought close enough to the current collector to be inelectrical communication, as well as the rate of the redox reaction onceit is in electrical communication with the current collector. In someinstances, the transport of ions across the ionically conductingmembrane may rate-limit the cell reaction. Thus the rate of charge ordischarge of the flow battery, or the power to energy ratio, may berelatively low. The number of battery cells or total area of theseparators or electroactive zones and composition and flow rates of theflowable redox compositions can be varied to provide sufficient powerfor any given application.

In some embodiments, the redox flow energy storage device may beconstructed and arranged such that a first electrode current collectorof a first polarity is at least partially surrounded by a secondelectrode current collector of a second, opposite polarity. In such anarrangement, a first electroactive zone of a first polarity may be atleast partially surrounded by a second electroactive zone of a second,opposite polarity. As used herein, a first electroactive zone is “atleast partially surrounded” by a second electroactive zone if a closedloop can be drawn around the first electroactive zone through only thesecond electroactive zone, and does not imply that the firstelectroactive zone is necessarily completely encapsulated by the secondelectroactive zone.

FIGS. 1B and 1C include cross-sectional schematic illustrations of onesuch redox flow energy storage device 500. In FIGS. 1B and 1C, device500 includes a positive electrode current collector 510 disposed withina negative electrode current collector 520. As illustrated in FIGS. 1Band 1C, the negative electrode current collector is substantiallycylindrical, comprising a cavity in which the positive electrode currentcollector is disposed. In some embodiments, as illustrated in FIGS. 1Band 1C, a first electrode current collector can be substantiallyconcentrically disposed within a second electrode current collector suchthat their longitudinal axes (indicated by dashed line 505 in FIG. 1C)coincide. It should be understood that, in some instances, thelongitudinal axes of the first and second electrode current collectorsmay not coincide. The positive and negative electrode current collectorscan be separated by an ion permeable medium 530 to define a positiveelectroactive zone 515 and a negative electroactive zone 525 that atleast partially surrounds the positive electroactive zone. While the setof embodiments illustrated in FIGS. 1B and 1C includes a positiveelectrode current collector and a positive electroactive zone at leastpartially surrounded by a negative electrode current collector and anegative electroactive zone, it should be understood that, in someembodiments, the polarities of the electrode current collectors andelectroactive zones can be reversed such that a negative electrodecurrent collector and a negative electroactive zone are at leastpartially surrounded by a positive electrode current collector and apositive electroactive zone.

Flowable redox material (e.g., an ionic solution, a semi-solid, or acondensed ion-storing electroactive material) can flow through thepositive electroactive zone and/or the negative electroactive zone, insome cases. Positive electrode flowable material can enter the positiveelectroactive zone 515 in the direction indicated by arrows 560 in FIG.1C. The positive electrode flowable material can flow through theelectroactive zone and can exit at the upper location of theelectroactive zone in the direction indicated by arrows 565. Similarly,negative electrode flowable material can enter the negativeelectroactive zone 525 in the direction indicated by arrows 570.Negative electrode material can flow through the negative electroactivezone and exit at the upper location of the electroactive zone in thedirection indicated by arrow 575. As noted with respect to FIG. 1A, theillustration of the direction of flow is arbitrary in FIG. 1C. Duringoperation, the positive and negative electroactive materials can undergoreduction and oxidation. Ions 590 can move across ion permeable medium530 (e.g., a membrane) and electrons can flow through an externalcircuit 580 to generate current.

In some embodiments, the positive and/or negative electrode currentcollector may include a plurality of surface features (e.g.,protrusions). In some instances, the surface features can includeprotrusions (e.g., posts, fins, baffles, etc.) that extend from asurface of the electrode current collector into an electroactive region.For example, FIG. 1D includes a schematic illustration of an electrodecurrent collector 600 comprising a plurality of posts 651 protrudingfrom surface 620 of the current collector. In some embodiments, theprotrusions can be electrically conductive. In some embodiments, theprotrusions can comprise modified tesla structures as illustrated inFIG. 1E and described in Hong et al., Lab on a chip, 4(2):109-13, 2004,which is incorporated herein by reference in its entirety. Suchstructures may be useful in enhancing mixing, as current collectors thatincrease half-cell conductivity, and/or in providing mechanical supportfor the separator. The presence of electrically conductive protrusionscan enhance the amount of electrically conductive surface area of thecurrent collector, relative to an amount of electrically conductivesurface area that would be present in the absence of the protrusions.

The protrusions can be, in some instances, constructed and arranged toenhance the circulation of the flowable redox composition. In someembodiments, the protrusions may be constructed and arranged to at leastpartially direct the flow of the flowable redox composition within anelectroactive region. In some instances, the surface features caninclude features formed into the bulk of the electrode current collector(e.g., channels) which may, in some cases, at least partially direct theflow of fluid within an electroactive region.

The electrode current collectors can, in some cases, include surfacefeatures that force fluid along the longitudinal axis of the redox flowenergy storage device (e.g., when the electrode current collector ismoved, such as when it is rotated). For example, in some embodiments, atleast one of the electrode current collectors can comprise a pluralityof undulations formed in the shape of a helix that forms a threading onat least a portion of a electrode current collector. One such example isillustrated in FIGS. 1F-1G. Such threading can be similar to those thatwould be observed along the exterior of a threaded screw or bolt oralong the interior of a threaded nut. The helical undulations in thecurrent collector may transport the flowable ion-storing redoxcomposition along the longitudinal axis of the current collector as thecurrent collector is rotated around its longitudinal axis. In somecases, both the positive and the negative electrode current collectorcan include threading over at least a part of their surfaces proximatean electroactive region. The threading on the positive and negativeelectrode current collectors may be of the same or different“handedness.” One of ordinary skill in the art would understand themeaning of handedness in this context as being similar to the concept ofhandedness as applied to screws and other threaded materials.

The surface features outlined above can provide one or more advantagesto the energy storage device. For example, in some embodiments (e.g.,when the protrusions produce a threaded surface on a current collector)the path over which a flowable ion-storing redox composition travels canbe relatively long, compared to the path that would be traveled in theabsence of the surface features. In addition, the presence of surfacefeatures on a current collector can increase the surface area of thecurrent collector that is exposed to a flowable ion-storing redoxcomposition, thus enhancing device performance. The presence of thesurface features may also allow the current collectors to be spacedrelatively closely. Especially close spacing between the two currentcollectors (and, hence, between each current collector and theseparation medium) can be achieved, for example, when the first andsecond current collectors each include threaded surfaces that arearranged such that they oppose each other.

In some embodiments, at least one of the positive or negative flowableredox compositions includes a semi-solid or a condensed ion-storingliquid electroactive material.

During discharging operation, the difference in electrochemicalpotentials of the positive and negative electrode of the redox flowdevice can produces a voltage difference between the positive andnegative electrodes; this voltage difference would produce an electriccurrent if the electrodes were connected in a conductive circuit. Insome embodiments, during discharging, a new volume of charged flowablesemi-solid or condensed liquid ion-storing composition is transportedfrom a charged composition storage tank into the electroactive zone. Insome embodiments, during discharging, the discharged or depletedflowable semi-solid or condensed liquid ion-storing composition can betransported out of the electroactive zone and stored in a dischargedcomposition storage receptacle until the end of the discharge.

During charging operation, the electrode containing flowable redoxcomposition can be run in reverse, either electrochemically andmechanically. In some embodiments, the depleted flowable semi-solid orcondensed liquid ion-storing composition can be replenished bytransporting the depleted redox composition out of the electroactivezone and introducing fully charged flowable semi-solid or condensedliquid ion-storing composition into the electroactive zone. This couldbe accomplished by using a fluid transportation device such as a pump.In some other embodiments, an opposing voltage difference can be appliedto the flowable redox energy storage device to drive electronic currentand ionic current in a direction opposite to that of discharging, toreverse the electrochemical reaction of discharging, thus charging theflowable redox composition of the positive and negative electrodes. Insome specific embodiments, during charging, discharged or depletedflowable semi-solid or condensed liquid ion-storing composition ismechanically transported into the electroactive zone to be charged underthe opposing voltage difference applied to the electrodes. In somespecific embodiments, the charged flowable semi-solid or condensedliquid ion-storing composition is transported out of the electroactivezone and stored in a charged composition storage receptacle until theend of the charge. The transportation can be accomplished by using afluid transportation device such as a pump.

One distinction between a conventional flow battery anolyte andcatholyte and the ion-storing solid or liquid phases as exemplifiedherein is the molar concentration or molarity of redox species in thestorage compound. For example, conventional anolytes or catholytes thathave redox species dissolved in aqueous solution may be limited inmolarity to typically 2M to 8M concentration. Highly acidic solutionsmay be necessary to reach the higher end of this concentration range. Bycontrast, any flowable semi-solid or condensed liquid ion-storing redoxcomposition as described herein may have, when taken in moles per literor molarity, at least 10M concentration of redox species, preferably atleast 12M, still preferably at least 15M, and still preferably at least20M. The electrochemically active material can be an ion storagematerial or any other compound or ion complex that is capable ofundergoing Faradaic reaction in order to store energy. The electroactivematerial can also be a multiphase material including the above-describedredox-active solid or liquid phase mixed with a non-redox-active phase,including solid-liquid suspensions, or liquid-liquid multiphasemixtures, including micelles or emulsions having a liquid ion-storagematerial intimately mixed with a supporting liquid phase. In the case ofboth semi-solid and condensed liquid storage compounds for the flowableion-storing redox compositions, systems that utilize various workingions are contemplated, including aqueous systems in which H⁺ or OH⁻ arethe working ions, nonaqueous systems in which Li⁺, Na⁺, or other alkaliions are the working ions, even alkaline earth working ions such as Ca²⁺and Mg²⁺, or Al³⁺. In each of these instances, a negative electrodestorage material and a positive electrode storage material may berequired, the negative electrode storing the working ion of interest ata lower absolute electrical potential than the positive electrode. Thecell voltage can be determined approximately by the difference inion-storage potentials of the two ion-storage electrode materials.

In some embodiments, the flowable redox composition is electricallyconductive. The flowable redox composition can be electricallyconductive while in its flowing and/or non-flowing state. In someembodiments the flowable redox composition (which can be, for example, asemi-solid or a condensed liquid ion-storing electroactive material) hasan electrical conductivity of at least about 10⁻⁶ S/cm, at least about10⁻⁵ S/cm, at least about 10⁻⁴ S/cm, or at least about 10⁻³ S/cm whileit is flowing and while it is at the temperature at which the energystorage device is operated (e.g., at least one temperature between about−50° C. and about +50° C.). In some embodiments, said composition has anelectronic conductivity in its non-flowing state of at least about 10⁻⁶S/cm, at least about 10⁻⁵ S/cm, at least about 10⁻⁴ S/cm, or at leastabout 10⁻³ S/cm at the temperature at which the energy storage device isoperated (e.g., at least one temperature between about −50° C. and about+50° C.). As specific examples, the flowable redox composition cancomprise a condensed liquid ion-storing electroactive material havingany of the electrical conductivities described herein (while flowingand/or while stationary). In some embodiments, the flowable redoxcomposition comprises a semi-solid, wherein the mixture of the liquidand solid phases, when measured together, has any of the electricalconductivities described herein (while flowing and/or while stationary).

Systems employing both negative and positive ion-storage materials areparticularly advantageous because there are no additionalelectrochemical byproducts in the cell. Both the positive and negativeelectrodes materials are insoluble in the flow electrolyte and theelectrolyte does not become contaminated with electrochemicaldecomposition products that must be removed and regenerated. Inaddition, systems employing both negative and positive lithiumion-storage materials are particularly advantageous when usingnon-aqueous electrochemical compositions.

In some embodiments, the flowable semi-solid or condensed liquidion-storing redox compositions include materials proven to work inconventional, solid lithium-ion batteries. In some embodiments, thepositive flowable electroactive materials contains lithium positiveelectroactive materials and the lithium cations are shuttled between thenegative electrode and positive electrode, intercalating into solid,host particles suspended in a liquid electrolyte.

In some embodiments at least one of the energy storage electrodesincludes a condensed ion-storing liquid of a redox-active compound,which may be organic or inorganic, and includes but is not limited tolithium metal, sodium metal, lithium-metal alloys, gallium and indiumalloys with or without dissolved lithium, molten transition metalchlorides, thionyl chloride, and the like, or redox polymers andorganics that are liquid under the operating conditions of the battery.Such a liquid form may also be diluted by or mixed with another,non-redox-active liquid that is a diluent or solvent, including mixingwith such a diluents to form a lower-melting liquid phase. However,unlike a conventional flow cell catholyte or anolyte, the redox activecomponent will comprise by mass at least 10% of the total mass of theflowable electrolyte, and preferably at least 25%.

In some embodiments, the redox-active electrode material, whether usedas a semi-solid or a condensed liquid format as defined above, comprisesan organic redox compound that stores the working ion of interest at apotential useful for either the positive or negative electrode of abattery. Such organic redox-active storage materials include “p”-dopedconductive polymers such as polyaniline or polyacetylene basedmaterials, polynitroxide or organic radical electrodes (such as thosedescribed in: H. Nishide et al., Electrochim. Acta, 50, 827-831, (2004),and K. Nakahara, et al., Chem. Phys. Lett., 359, 351-354 (2002)),carbonyl based organics, and oxocarbons and carboxylate, includingcompounds such as Li₂C₆O₆, Li₂C₈H₄O₄, and Li₂C₆H₄O₄ (see for example M.Armand et al., Nature Materials, DOI: 10.1038/nmat2372).

In some embodiments the redox-active electrode material comprises a solor gel, including for example metal oxide sols or gels produced by thehydrolysis of metal alkoxides, amongst other methods generally known as“sol-gel processing.” Vanadium oxide gels of composition V_(x)O_(y) areamongst such redox-active sol-gel materials.

Other suitable positive active materials include solid compounds knownto those skilled in the art as those used in NiMH (Nickel-Metal Hydride)Nickel Cadmium (NiCd) batteries. Still other positive electrodecompounds for Li storage include those used in carbon monofluoridebatteries, generally referred to as CF_(x), or metal fluoride compoundshaving approximate stoichiometry MF₂ or MF₃ where M comprises Fe, Bi,Ni, Co, Ti, V. Examples include those described in H. Li, P. Balaya, andJ. Maier, Li-Storage via Heterogeneous Reaction in Selected Binary MetalFluorides and Oxides, Journal of The Electrochemical Society, 151 [11]A1878-A1885 (2004), M. Bervas, A. N. Mansour, W.-S. Woon, J. F.Al-Sharab, F. Badway, F. Cosandey, L. C. Klein, and G. G. Amatucci,“Investigation of the Lithiation and Delithiation Conversion Mechanismsin a Bismuth Fluoride Nanocomposites”, J. Electrochem. Soc., 153, A799(2006), and I. Plitz, F. Badway, J. Al-Sharab, A. DuPasquier, F.Cosandey and G. G. Amatucci, “Structure and Electrochemistry ofCarbon-Metal Fluoride Nanocomposites Fabricated by a Solid State RedoxConversion Reaction”, J. Electrochem. Soc., 152, A307 (2005).

As another example, fullerenic carbon including single-wall carbonnanotubes (SWNTs), multiwall carbon nanotubes (MWNTs), or metal ormetalloid nanowires may be used as ion-storage materials. One example isthe silicon nanowires used as a high energy density storage material ina report by C. K. Chan, H. Peng, G. Liu, K. McIlwrath, X. F. Zhang, R.A. Huggins, and Y. Cui, High-performance lithium battery anodes usingsilicon nanowires, Nature Nanotechnology, published online 16 Dec. 2007;doi:10.1038/nnano.2007.411.

Exemplary electroactive materials for the positive electrode in alithium system include the general family of ordered rocksalt compoundsLiMO₂ including those having the α-NaFeO₂ (so-called “layeredcompounds”) or orthorhombic-LiMnO₂ structure type or their derivativesof different crystal symmetry, atomic ordering, or partial substitutionfor the metals or oxygen. M comprises at least one first-row transitionmetal but may include non-transition metals including but not limited toAl, Ca, Mg, or Zr. Examples of such compounds include LiCoO₂, LiCoO₂doped with Mg, LiNiO₂, Li(Ni, Co, Al)O₂ (known as “NCA”) and Li(Ni, Mn,Co)O₂ (known as “NMC”). Other families of exemplary electroactivematerials includes those of spinel structure, such as LiMn₂O₄ and itsderivatives, “high voltage spinels” with a potential vs. Li/Li⁺ thatexceeds 4.3V including but not limited to LiNi_(0.5)Mn_(1.5)O₄,so-called “layered-spinel nanocomposites” in which the structureincludes nanoscopic regions having ordered rocksalt and spinel ordering,olivines LiMPO₄ and their derivatives, in which M comprises one or moreof Mn, Fe, Co, or Ni, partially fluorinated compounds such as LiVPO₄F,other “polyanion” compounds as described below, and vanadium oxidesV_(x)O_(y) including V₂O₅ and V₆O₁₁.

In one or more embodiments the active material comprises a transitionmetal polyanion compound, for example as described in U.S. Pat. No.7,338,734. In one or more embodiments the active material comprises analkali metal transition metal oxide or phosphate, and for example, thecompound has a composition A_(x)(M′_(1-a)M″_(a))_(y)(XD₄)_(z),A_(x)(M′_(1-a)M″_(a))_(y)(DXD₄)_(z), orA_(x)(M′_(1-a)M″_(a))_(y)(X₂D₇)_(z), and have values such that x, plusy(1-a) times a formal valence or valences of M′, plus ya times a formalvalence or valence of M″, is equal to z times a formal valence of theXD₄, X₂D₇, or DXD₄ group; or a compound comprising a composition(A_(1-a)M″_(a))_(x)M′_(y)(XD₄)_(z), (A_(1-a)M″_(a))_(x)M′_(y)(DXD₄)z(A_(1-a)M″_(a))_(x)M′_(y)(X₂D₇)_(z) and have values such that (1-a)_(x)plus the quantity ax times the formal valence or valences of M″ plus ytimes the formal valence or valences of M′ is equal to z times theformal valence of the XD₄, X₂D₇ or DXD₄ group. In the compound, A is atleast one of an alkali metal and hydrogen, M′ is a first-row transitionmetal, X is at least one of phosphorus, sulfur, arsenic, molybdenum, andtungsten, M″ any of a Group IIA, IIIA, IVA, VA, VIA, VILA, VIIIA, IB,IIB, IIIB, IVB, VB, and VIB metal, D is at least one of oxygen,nitrogen, carbon, or a halogen. The positive electroactive material canbe an olivine structure compound LiMPO₄, where M is one or more of V,Cr, Mn, Fe, Co, and Ni, in which the compound is optionally doped at theLi, M or O-sites. Deficiencies at the Li-site are compensated by theaddition of a metal or metalloid, and deficiencies at the O-site arecompensated by the addition of a halogen. In some embodiments, thepositive active material comprises a thermally stable,transition-metal-doped lithium transition metal phosphate having theolivine structure and having the formula (Li_(1-x)Z_(x))MPO₄, where M isone or more of V, Cr, Mn, Fe, Co, and Ni, and Z is a non-alkali metaldopant such as one or more of Ti, Zr, Nb, Al, or Mg, and x ranges from0.005 to 0.05.

In other embodiments, the lithium transition metal phosphate materialhas an overall composition of Li_(1-x-z)M_(1+z)PO₄, where M comprises atleast one first row transition metal selected from the group consistingof Ti, V, Cr, Mn, Fe, Co and Ni, where x is from 0 to 1 and z can bepositive or negative. M includes Fe, z is between about 0.15 and −0.15.The material can exhibit a solid solution over a composition range of0<x<0.15, or the material can exhibit a stable solid solution over acomposition range of x between 0 and at least about 0.05, or thematerial can exhibit a stable solid solution over a composition range ofx between 0 and at least about 0.07 at room temperature (22-25° C.). Thematerial may also exhibit a solid solution in the lithium-poor regime,e.g., where x≧0.8, or x≧0.9, or x≧0.95.

In some embodiments the redox-active electrode material comprises ametal salt that stores an alkali ion by undergoing a displacement orconversion reaction. Examples of such compounds include metal oxidessuch as CoO, Co₃O₄, NiO, CuO, MnO, typically used as a negativeelectrode in a lithium battery, which upon reaction with Li undergo adisplacement or conversion reaction to form a mixture of Li₂O and themetal constituent in the form of a more reduced oxide or the metallicform. Other examples include metal fluorides such as CuF₂, FeF₂, FeF₃,BiF₃, CoF₂, and NiF₂, which undergo a displacement or conversionreaction to form LiF and the reduced metal constituent. Such fluoridesmay be used as the positive electrode in a lithium battery. In otherembodiments the redox-active electrode material comprises carbonmonofluoride or its derivatives. In some embodiments the materialundergoing displacement or conversion reaction is in the form ofparticulates having on average dimensions of 100 nanometers or less. Insome embodiments the material undergoing displacement or conversionreaction comprises a nanocomposite of the active material mixed with aninactive host, including but not limited to conductive and relativelyductile compounds such as carbon, or a metal, or a metal sulfide.

In some embodiments the semi-solid flow battery is a lithium battery,and the negative electrode active compound comprises graphite, graphiticboron-carbon alloys, hard or disordered carbon, lithium titanate spinel,or a solid metal or metal alloy or metalloid or metalloid alloy thatreacts with lithium to form intermetallic compounds, including themetals Sn, Bi, Zn, Ag, and Al, and the metalloids Si and Ge. In someembodiments, Li₄Ti₅O₁₂ can be included as an electrode active material(e.g., a negative electrode active material).

Exemplary electroactive materials for the negative electrode in the caseof a lithium working ion include graphitic or non-graphitic carbon,amorphous carbon, or mesocarbon microbeads; an unlithiated metal ormetal alloy, such as metals including one or more of Ag, Al, Au, B, Ga,Ge, In, Sb, Sn, Si, or Zn, or a lithiated metal or metal alloy includingsuch compounds as LiAl, Li₉Al₄, Li₃Al, LiZn, LiAg, Li₁₀Ag₃, Li₅B₄,Li₇B₆, Li₁₂Si₇, Li₂₁Si₈, Li₁₃Si₄, Li₂₁Si₅, Li₅Sn₂, Li₁₃Sn₅, Li₇Sn₂,Li₂₂Sn₅, Li₂Sb, Li₃Sb, LiBi, or Li₃Bi, or amorphous metal alloys oflithiated or non-lithiated compositions.

The current collector can be electronically conductive and should beelectrochemically inactive under the operation conditions of the cell.Typical current collectors for lithium cells include copper, aluminum,or titanium for the negative current collector and aluminum for thepositive current collector, in the form of sheets or mesh, or anyconfiguration for which the current collector may be distributed in theelectrolyte and permit fluid flow. Selection of current collectormaterials is well-known to those skilled in the art. In someembodiments, aluminum is used as the current collector for positiveelectrode. In some embodiments, copper is used as the current collectorfor negative electrode. In other embodiments, aluminum is used as thecurrent collector for negative electrode.

In some embodiments, the negative electrode can be a conventionalstationary electrode, while the positive electrode includes a flowableredox composition. In other embodiments, the positive electrode can be aconventional stationary electrode, while the negative electrode includesa flowable redox composition.

In some embodiments, the semi-solid flow cells of the present inventionuse Li⁺ or Na⁺ as the working ion and comprise an aqueous electrolyte.Although the use of aqueous electrolytes can, in some cases, require theuse of lower potentials (to avoid the electrolytic decomposition ofwater) than can be used with some nonaqueous systems (e.g., conventionallithium ion systems using alkyl carbonate electrolyte solvents), theenergy density of a semi-solid aqueous flow battery can be much greaterthan that of a conventional aqueous solution flow cell (e.g., vanadiumredox or zinc-bromine chemistry) due to the much greater density of ionstorage that is possible in the solid phase of a semi-solid catholyte oranolyte. Aqueous electrolytes are typically less expensive thannonaqueous electrolytes and can lower the cost of the flow battery,while typically also having higher ionic conductivity. In addition,aqueous electrolyte systems can be less prone to formation of insulatingSEIs on the conductive solid phases used in the catholyte or anolyte, orcurrent collectors, which can increase the impedance of the flowbattery.

The following non-limiting examples of aqueous systems show that a broadrange of cathode-active materials, anode-materials, current collectormaterials, electrolytes, and combinations of such components may be usedin the semi-solid aqueous flow batteries of this set of embodiments.

In some embodiments, oxides of general formula A_(x)M_(y)O_(z) may beused as ion storage compounds in an aqueous semi-solid flow cell,wherein A comprises a working ion that may be one or more of Na, Li, K,Mg, Ca, and Al; M comprises a transition metal that changes its formalvalence state as the working ion is intercalated or deintercalated fromthe compound; O corresponds to oxygen; x can have a value of 0 to 10; ycan have a value of 1 to 3; and z can have a value of 2 to 7.

The aqueous or nonaqueous semi-solid flow cells may also comprise, asthe semi-solid ion storage electrode, one or more lithium metal“polyanion” compounds, including but not limited to compounds describedin U.S. Pat. No. 7,338,734, to Chiang et al. which is incorporatedherein by reference in its entirety for all purposes. Such compoundsinclude the compositions (A)_(x)(M′_(1-a)M″_(a))_(y)(XD₄)_(z),A_(x)(M′_(1-a)M″_(a))_(y)(DXD₄)_(z), orA_(x)(M′_(1-a)M″_(a))_(y)(X₂D₇)_(z), wherein A is at least one of analkali metal or hydrogen, M′ is a first-row transition metal, X is atleast one of phosphorus, sulfur, arsenic, boron, aluminum, silicon,vanadium, molybdenum and tungsten, M″ is any of a Group IIA, IIIA, IVA,VA, VIA, VIIA, VIIIA, IB, IIB, IIIB, IVB, VB, and VIB metal, D is atleast one of oxygen, nitrogen, carbon, or a halogen, 0≦a≦0.1, x is equalto or greater than 0, y and z are greater than 0 and have values suchthat x, plus y(1-a) times a formal valence or valences of M′, plus yatimes a formal valence or valence of M″, is equal to z times a formalvalence of the XD₄, X₂D₇, or DXD₄ group. In some embodiments, thecompound crystallizes in an ordered or partially disordered structure ofthe olivine (A_(x)MXO₄), NASICON (A_(x)(M′,M″)₂(XO₄)₃), VOPO₄,LiFe(P₂O₇) or Fe₄(P₂O₇)₃ structure-types, and has a molar concentrationof the metals (M′+M″) relative to the concentration of the elements Xthat exceeds the ideal stoichiometric ratio y/z of the prototypecompounds by at least 0.0001.

Other such compounds comprise the compositions(A_(1-a)M″_(a))_(x)M′_(y)(XD₄)_(z), (A_(1-a)M″_(a))_(x)M′_(y)(DXD₄)_(z),or (A_(1-a)M″_(a))_(x)M′_(y)(X₂D₇)_(z), wherein A is at least one of analkali metal or hydrogen; M′ is a first-row transition metal; X is atleast one of phosphorus, sulfur, arsenic, boron, aluminum, silicon,vanadium, molybdenum and tungsten; M″ any of a Group IIA, IIIA, IVA, VA,VIA, VIIA, VIIIA, IB, IIB, IIIB, IVB, VB, and VIB metal; D is at leastone of oxygen, nitrogen, carbon, or a halogen; 0≦a≦0.1; and x, y, and zare greater than zero and have values such that (1-a)_(x) plus thequantity ax times the formal valence or valences of M″ plus y times theformal valence or valences of M′ is equal to z times the formal valenceof the XD₄, X₂D₇ or DXD₄ group. In some of these embodiments, thecompound crystallizes in an ordered or partially disordered structure ofthe olivine (A_(x)MXO₄), NASICON (A_(x)(M′,M″)₂(XO₄)₃), VOPO₄,LiFe(P₂O₇) or Fe₄(P₂O₇)₃ structure-types, and has a molar concentrationof the metals (M′+M″) relative to the concentration of the elements Xthat exceeds the ideal stoichiometric ratio y/z of the prototypecompounds by at least 0.0001.

Still other such compounds comprise the compositions(A_(b-a)M″_(a))_(x)M′_(y)(XD₄)_(z), (A_(b-a)M″_(a))_(x)M′_(y)(DXD₄)_(z),or (A_(b-a)M″_(a))_(x)M′_(y)(X₂D₇)_(z), wherein A is at least one of analkali metal or hydrogen; M′ is a first-row transition metal; X is atleast one of phosphorus, sulfur, arsenic, boron, aluminum, silicon,vanadium, molybdenum and tungsten; M″ any of a Group IIA, IIIA, IVA, VA,VIA, VIIA, VIIIA, IB, IIB, IIIB, IVB, VB, and VIB metal; D is at leastone of oxygen, nitrogen, carbon, or a halogen; 0≦a≦0.1; a≦b≦1; and x, y,and z are greater than zero and have values such that (b-a)x plus thequantity ax times the formal valence or valences of M″ plus y times theformal valence or valences of M′ is equal to z times the formal valenceof the XD₄, X₂D₇ or DXD₄ group. In some of these embodiments, thecompound crystallizes in an ordered or partially disordered structure ofthe olivine (A_(x)MXO₄), NASICON (A_(x)(M′,M″)₂(XO₄)₃), VOPO₄,LiFe(P₂O₇) or Fe₄(P₂O₇)₃ structure-types, and has a molar concentrationof the metals (M′+M″) relative to the concentration of the elements Xthat exceeds the ideal stoichiometric ratio y/z of the prototypecompounds by at least 0.0001.

Rechargeable lithium batteries using an aqueous electrolyte have beendescribed by W. Li, J. R. Dahn, and D. S. Wainwright (Science, vol. 264,p. 1115, 20 May 1994). They demonstrated a rechargeable system in whichboth the cathode and anode are lithium intercalation compounds, beingLiMn₂O₄ and VO₂ (B) respectively, and the electrolyte is a solution of 5M LiNO₃ and 0.001 M LiOH in water, with a cell voltage of about 1.5V.Other aqueous rechargeable lithium batteries include the followingcombinations of cathode/anode: Li(Ni_(1-x)Co_(x))O₂/LiV₃O₈,LiCoO₂/LiV₃O₈, LiMn₂O₄/TiP₂O₇, LiMn₂O₄/LiTi₂(PO₄)₃,Li(Ni_(0.33)Mn_(0.33)Co_(0.33))O₂/Li_(x)V₂O₅, V₂O₅/Li_(x)V₂O₅,LiMn₂O₄/Li_(x)V₂O₅, LiMn₂O₄/NaTi₂(PO₄)₃, LiMn₂O₄/Li₃Fe₂(PO₄)₃,LiMn₂O₄/LiFeP₂O₇, LiMn₂O₄/LiFe₄(P₂O₇)₃, LiCoO₂/C, Li_(0.5)Mn₂O₄/LiCoO₂,γ-MnO₂/Zn, and TiO₂ (anatase)/Zn. The semi-solid flow batteriesdescribed herein can include the use of any one or more of thesecathode-active materials with any one or more of the anode-activematerials. Electrode conductive additives and binders, current collectormaterials, current collector coatings, and electrolytes that can be usedin such non-flow systems (as described herein) can also be used in thesemi-solid flow batteries described herein.

In some embodiments, the flow cell can include an aqueous positiveelectrode active material comprising a material of the general formulaLi_(x)Fe_(y)P_(a)O_(z), (wherein, for example, x can be between about0.5 and about 1.5, y can be between about 0.5 and about 1.5, a can bebetween about 0.5 and about 1.5, and z can be between about 3 and about5), and a negative electrode active material comprising a material ofthe general formula Li_(x′)Ti_(y′)O_(z′), (wherein, for example, x′ canbe between about 3 and about 5, y′ can be between about 4 and about 6,and z′ can be between about 9 and about 15 or between about 11 and about13). As a specific example, in some embodiments, the negative electrodeactive material can comprise LiFePO₄ and the positive electrode activematerial can comprise Li₄Ti₅O₁₂. In some embodiments, the positiveand/or negative electrode active materials can include cation or aniondoped derivatives of these compounds.

Other specific combinations of electrode active materials that can beused in aqueous flow cells (listed here as anode/cathode pairs) include,but are not limited to, LiV₃O₈/LiCoO₂; LiV₃O₈/LiNiO₂; LiV₃O₈/LiMn₂O₄;and C/Na_(0.44)MnO₂.

Sodium can be used as the working ion in conjunction with an aqueouselectrolyte and cathode-active or anode active compounds thatintercalate sodium at suitable potentials, or that store sodium bysurface adsorption and the formation of an electrical double layer as inan electrochemical capacitor or by surface adsorption accompanied bycharge transfer. Materials for such systems have been described in USPatent Application US 2009/0253025, by J. Whitacre, for use inconventional (non-flow type) secondary batteries. The semi-solid flowbatteries described herein can use one or more of the cathode-activematerials, anode-active materials, electrode conductive additives andbinders, current collector materials, current collector coatings, andelectrolytes considered in such non-flow systems. One or moreembodiments described herein can incorporate these materials insemi-solid flow batteries.

Cathode active materials that store sodium and can be used in an aqueouselectrolyte system include, but are not limited to, layered/orthorhombicNaMO₂ (birnessite), cubic spinel λ-MnO₂ based compounds, Na₂M₃O₇,NaMPO₄, NaM₂(PO₄)₃, Na₂ MPO₄F, and tunnel-structured Na_(0.44)MO₂, whereM is a first-row transition metal. Specific examples include NaMnO₂,Li_(x)Mn₂O₄ spinel into which Na is exchanged or stored,Li_(x)Na_(y)Mn₂O₄, Na_(y)Mn₂O₄, Na₂Mn₃O₇, NaFePO₄, Na₂FePO₄F, andNa_(0.44)MnO₂. Anode materials can include materials that store sodiumreversibly through surface adsorption and desorption, and include highsurface area carbons such as activated carbons, graphite, mesoporouscarbon, carbon nanotubes, and the like. They also may comprise highsurface area or mesoporous or nanoscale forms of oxides such as titaniumoxides, vanadium oxides, and compounds identified above as cathodematerials but which do not intercalate sodium at the operatingpotentials of the negative electrode.

Current collector materials can be selected to be stable at theoperating potentials of the positive and negative electrodes of the flowbattery. In nonaqueous lithium systems the positive current collectormay comprise aluminum, or aluminum coated with conductive material thatdoes not electrochemically dissolve at operating potentials of 2.5-5Vwith respect to Li/Li⁺. Such materials include Pt, Au, Ni, conductivemetal oxides such as vanadium oxide, and carbon. The negative currentcollector may comprise copper or other metals that do not form alloys orintermetallic compounds with lithium, carbon, and coatings comprisingsuch materials on another conductor.

In aqueous Na⁺ and Li⁺ flow batteries the positive current collector maycomprise stainless steel, nickel, nickel-chromium alloys, aluminum,titanium, copper, lead and lead alloys, refractory metals, and noblemetals. The negative current collector may comprise stainless steel,nickel, nickel-chromium alloys, titanium, lead oxides, and noble metals.In some embodiments, the current collector comprises a coating thatprovides electronic conductivity while passivating against corrosion ofthe metal. Examples of such coatings include, but are not limited to,TiN, CrN, C, CN, NiZr, NiCr, Mo, Ti, Ta, Pt, Pd, Zr, W, FeN, and CoN.Electrolytes used in aqueous semi-solid flow cells may comprise analkaline or alkaline earth salt dissolved in water to a concentration of0.1M to 10M. The salt used may comprise alkali or alkaline earth metalsother than the ion species stored in the intercalation electrode. Thusfor lithium and sodium storing electrodes, the electrolyte may containA₂SO₄, ANO₃, AClO₄, A₃PO₄, A₂CO₃, ACl, ANO₃, and AOH, where A comprisesLi, Na, both Li and Na, or K. Alkaline earth salts include but are notlimited to CaSO₄, Ca(NO₃)₂, Ca(ClO₄)₂, CaCO₃, Ca(OH)₂, MgSO₄, Mg(NO₃)₂,Mg(ClO₄)₂, MgCO₃, and Mg(OH)₂. The pH of an aqueous electrolyte may beadjusted using methods known to those of ordinary skill in the art, forexample by adding OH containing salts to raise pH, or acids to lower pH,in order to adjust the voltage stability window of the electrolyte or toreduce degradation by proton exchange of certain active materials.

In some embodiments the redox-active compound is present as a nanoscale,nanoparticle, or nanostructured form. This can facilitate the formationof stable liquid suspensions of the storage compound, and improves therate of reaction when such particles are in the vicinity of the currentcollector. The nanoparticulates may have equiaxed shapes or have aspectratios greater than about 3, including nanotubes, nanorods, nanowires,and nanoplatelets. Branched nanostructures such as nanotripods andnanotetrapods can also be used in some embodiments. Nanostructured ionstorage compounds may be prepared by a variety of methods includingmechanical grinding, chemical precipitation, vapor phase reaction,laser-assisted reactions, and bio-assembly. Bio-assembly methodsinclude, for example, using viruses having DNA programmed to template anion-storing inorganic compound of interest, as described in K. T. Nam,D. W. Kim, P. J. Yoo, C.-Y. Chiang, N. Meethong, P. T. Hammond, Y.-M.Chiang, A. M. Belcher, “Virus enabled synthesis and assembly ofnanowires for lithium ion battery electrodes,” Science, 312[5775],885-888 (2006).

In redox cells with a semi-solid flowable redox composition, too fine asolid phase can inhibit the power and energy of the system by “clogging”the current collectors. In one or more embodiments, the semi-solidflowable composition contains very fine primary particle sizes for highredox rate, but which are aggregated into larger agglomerates. Thus insome embodiments, the particles of solid redox-active compound in thepositive or negative flowable redox compositions are present in a porousaggregate of 1 micrometer to 500 micrometer average diameter.

The redox energy storage devices can include, in some embodiments, smallparticles that can comprise a lubricant such as, for example,fluoropolymers such as polytetrafluoroethylene (PTFE).

In some embodiments, acoustic energy is applied to the system to inhibitthe accumulation of particles of solid redox-active compound or anyother solid within the system. “Acoustic energy” is given its normalmeaning in the art, and is generally used to refer to an oscillation ofpressure transmitted through a medium. In one embodiment, the acousticenergy is applied to a semi-solid suspension, for example, used as thepositive and/or negative flowable redox composition in the inventiveredox flow energy storage devices. The application of acoustic energymay, for example, allow one to avoid undesirable states of particleaggregation in the flowable redox composition (e.g., a suspension), toavoid particle stratification and settling, to disrupt or inhibit theformation of solid-electrolyte interface (SEI) layers, to alter therheology of the suspension in-situ, among other reasons.

The acoustic energy can originate from any suitable source. In someembodiments, the acoustic energy source may be a discrete device (e.g.,removably attached to the energy storage device, positioned proximatethe energy storage device) or it may be monolithically integrated withthe energy storage device. For example, the acoustic energy can, in someembodiments, originate from a resonator. In one set of embodiments,acoustic energy can be provided by a piezoelectric or electrostrictiveactuator that is, for example, driven by an AC field.

Acoustic energy can be applied at any location within the flow cellsystem, including in a storage tank, at a segment of tubing or achannel, or within the redox flow energy storage device(s). For example,one or more piezoelectric actuators may be attached to the walls of astorage tank to control particle settling, in much the same manner thanan ultrasonic cleaning bath is constructed with a piezoelectric elementattached to the wall of the vessel. One or more acoustic energy sourcesmay be inserted in the tank itself, analogous to the use of anultrasonic “horn” to disperse particle suspensions in liquid. In someembodiments, at least one acoustic energy source is attached to pipes ortubing or joints between pipes or tubing that carry the semi-solidsuspension between storage tanks and the flow cell. In some cases, atleast one acoustic energy source is incorporated into an in-line sensorof the kind discussed herein. At least one acoustic energy source, insome embodiments, is attached to the outer surface(s) of the redox flowenergy storage device(s), or may be embedded within the layers of astack of redox flow energy storage devices, where it/they may be used tocontrol particle dispersion, settling, or suspension rheology.

In some embodiments, acoustic energy can be applied to the energystorage device at a frequency and/or level of energy selected to inhibitaccumulation of a solid in the energy storage device (e.g., within aflowable redox composition within the energy storage device). In someembodiments, the frequency and/or power of the acoustic energy sourcemay be tuned, using methods known to those of ordinary skill in the art,to, for example, enhance energy coupling to the suspension and/ormaintain low power consumption by the device. This can be accomplished,for example, by employing a source of acoustic energy that includes acontroller that allows for the application of a selected frequencyand/or amplitude of acoustic energy. In some embodiments, the acousticenergy source may be used to apply ultrasonic acoustic energy to theenergy storage device or a portion thereof.

In some embodiments, acoustic energy can be applied to the energystorage device at a frequency and/or level of energy selected to reducethe viscosity of a flowable redox composition within the energy storagedevice. In some embodiments, acoustic energy can be applied to theenergy storage device at a frequency and/or level of energy selected toreduce the viscosity of a flowable redox composition by at least about10%, at least about 20%, at least about 30%, at least about 40%, or atleast about 50%.

The ability to lower the viscosity of the flowable redox composition canbe particularly useful in small channels, constrictions, and other areaswhere transporting the redox composition can be difficult. In someembodiments, acoustic energy can be applied to a portion of a channelthrough which the flowable redox composition flows, the portion of thechannel having a smallest cross-sectional dimension of less than about 1cm, less than about 5 mm, less than about 1 mm, less than about 100micrometers, between about 10 micrometers and about 1 cm, between about10 micrometers and about 5 mm, or between about 10 micrometers and about1 mm. In some embodiments, the redox flow energy storage device isconstructed and arranged such that the flowable redox composition flowsthrough a substantially fluidically continuous channel, and the acousticenergy is applied to a portion of the channel having a smallestcross-sectional dimension that is less than about 0.5 times, less thanabout 0.25 times, less than about 0.1 times, less than about 0.05 times,or less than about 0.02 times the maximum cross sectional dimension ofthe channel. The “maximum cross-sectional dimension” of a channel, asused herein, refers to the largest cross-sectional distance between theboundaries of the channel, as measured perpendicular to the length ofthe channel (i.e., perpendicular to the direction of fluid flow).Likewise, the “minimum cross-sectional dimension” of a channel, as usedherein, refers to the smallest cross-sectional distance between theboundaries of the channel, as measured perpendicular to the length ofthe channel (i.e., perpendicular to the direction of fluid flow).

The ion-permeable medium through which ions are transported within theredox flow energy storage device can include any suitable medium capableof allowing ions to be passed through it. In some embodiments, theion-permeable medium can comprise a membrane. The membrane can be anyconventional membrane that is capable of ion transport. In one or moreembodiments, the membrane is a liquid-impermeable membrane that permitsthe transport of ions therethrough, namely a solid or gel ionicconductor. In other embodiments the membrane is a porous polymermembrane infused with a liquid electrolyte that allows for the shuttlingof ions between the anode and cathode electroactive materials, whilepreventing the transfer of electrons. In some embodiments, the membraneis a microporous membrane that prevents particles forming the positiveand negative electrode flowable compositions from crossing the membrane.Exemplary membrane materials include polyethyleneoxide (PEO) polymer inwhich a lithium salt is complexed to provide lithium conductivity, orNafion™ membranes which are proton conductors. For example, PEO basedelectrolytes can be used as the membrane, which is pinhole-free and asolid ionic conductor, optionally stabilized with other membranes suchas glass fiber separators as supporting layers. PEO can also be used asa slurry stabilizer, dispersant, etc. in the positive or negativeflowable redox compositions. PEO is stable in contact with typical alkylcarbonate-based electrolytes. This can be especially useful inphosphate-based cell chemistries with cell potential at the positiveelectrode that is less than about 3.6 V with respect to Li metal. Theoperating temperature of the redox cell can be elevated as necessary toimprove the ionic conductivity of the membrane.

In some embodiments, a carrier liquid is used to suspend and transportthe solid phase or condensed liquid of the flowable redox composition.The carrier liquid can be any liquid that can suspend and transport thesolid phase or condensed ion-storing liquid of the flowable redoxcomposition. By way of example, the carrier liquid can be water, a polarsolvent such as alcohols or aprotic organic solvents. Numerous organicsolvents have been proposed as the components of Li-ion batteryelectrolytes, notably a family of cyclic carbonate esters such asethylene carbonate, propylene carbonate, butylene carbonate, and theirchlorinated or fluorinated derivatives, and a family of acyclic dialkylcarbonate esters, such as dimethyl carbonate, diethyl carbonate,ethylmethyl carbonate, dipropyl carbonate, methyl propyl carbonate,ethyl propyl carbonate, dibutyl carbonate, butylmethyl carbonate,butylethyl carbonate and butylpropyl carbonate. Other solvents proposedas components of Li-ion battery electrolyte solutions includeγ-butyrolactone, dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, diethyl ether,sulfolane, methylsulfolane, acetonitrile, propiononitrile, ethylacetate, methyl propionate, ethyl propionate, dimethyl carbonate,tetraglyme, and the like. These nonaqueous solvents are typically usedas multicomponent mixtures, into which a salt is dissolved to provideionic conductivity. Exemplary salts to provide lithium conductivityinclude LiClO₄, LiPF₆, LiBF₄, lithium bis(pentafluorosulfonyl)imide(also referred to as LiBETI), lithium bis(trifluoromethane)sulfonimide(also referred to as LiTFSI), lithium bis(oxalato)borate (also referredto as LiBOB), and the like. As specific examples, the carrier liquid cancomprise 1,3-dioxolane mixed with lithium bis(pentafluorosulfonyl)imide,for example, in a mixture of about 70:30 by mass; an alkyl carbonatemixed with LiPF₆; LiPF₆ in dimethyl carbonate DMC (e.g., at a molarityof about 1 M); LiClO₄ in 1,3-dioxolane (e.g., at a molarity of about 2M); and/or a mixture of tratraglyme and lithium bis(pentafluorosulfonyl)imide (e.g., in a molar ratio of about 1:1).

In some embodiments, the carrier liquid used within a flowable redoxcomposition (e.g., to suspend and transport the solid phase within theflowable redox composition) and/or a salt included in the flowable redoxcomposition (e.g., in a semi-solid suspension used as a catholyte oranolyte in the semisolid flow cell) is selected for its ability toinhibit the formation of a solid-electrolyte interface (SEI). Theformation of SEI is a phenomenon known to those of ordinary skill in theart, and is normally present in, for example, primary and secondarylithium batteries. Formation of a thin and stable SEI on the electrodecan be desirable in conventional lithium-ion batteries, as it canprovide controlled passivation of the electrodes against oxidationreactions (at the positive electrode) or reduction reactions (at thenegative electrode) that, if allowed to continue, can consume workinglithium in the cell, increase the impedance of the electrodes, introducesafety issues, or degrade the electrolyte. However, in some embodimentsdescribed herein, formation of SEI can be undesirable. For example,formation of SEI on conductive particles in the semi-solid suspension oron the surfaces of the current collectors can decrease cell performance,as such films are generally electronically insulating, and can increasethe internal resistance of said flow cell. Thus it can be advantageousto select carrier liquids and/or salts that minimize SEI formation atthe working potential of the positive and/or negative flowable redoxcomposition (e.g., catholyte and/or anolyte). In some embodiments, thesame composition (e.g., carrier fluid, salt, and/or electroactive solidmaterial) is used in both the positive flowable redox composition andthe negative flowable redox composition, and is selected to have anelectrochemical stability window that includes the potentials at bothelectrodes or current collectors of the flow cell. In other embodiments,the components of the positive and negative flowable redox composition(e.g., carrier fluid, salt, and/or electroactive solid material) areseparately chosen and used to enhance the performance of the positiveand/or negative flowable redox compositions (and their respectivecurrent collectors). In such cases, the electrolyte phase of thesemi-solid cathode and anode may be separated in the flow cell by usinga separation medium (e.g., a separator membrane) that is partially orcompletely impermeable to the carrier liquids, while permitting faciletransport of the working ion between positive and negative flowableredox compositions. In this way, a first carrier liquid can be used inthe positive electroactive zone (e.g., in the positive flowable redoxcomposition), and a second, different carrier liquid can be used in thenegative electroactive zone (e.g., in the negative flowable redoxcomposition).

A variety of carrier liquids can be selected for advantageous use at thenegative and/or positive electrode of the flow cells described herein.In some embodiments, the carrier liquid compound includes 1 oxygen atom.For example, the carrier liquid may include an ether (e.g., an acyclicether, a cyclic ether) or a ketone (e.g., an acyclic ketone, a cyclicketone) in some embodiments. In some cases, the carrier liquid includesa symmetric acyclic ether such as, for example, dimethyl ether, diethylether, di-n-propyl ether, and diisopropyl ether. In some cases, thecarrier liquid includes an asymmetric acyclic ether such as, forexample, ethyl methyl ether, methyl n-propyl ether, isopropyl methylether, methyl n-butyl ether, isobutyl methyl ether, methyl s-butylether, methyl t-butyl ether, ethyl isopropyl ether, ethyl n-propylether, ethyl n-butyl ether, ethyl i-butyl ether, ethyl s-butyl ether,and ethyl t-butyl ether. In some cases, the carrier liquid includes acyclic ether including 5-membered rings such as, for example,tetrahydrofuran, 2-methyl tetrahydrofuran, 3-methyl tetrahydrofuran. Thecarrier liquid can include, in some embodiments, a cyclic etherincluding 6-membered rings such as, for example, tetrahydropyran,2-methyl tetrahydropyran, 3-methyl tetrahydropyran, 4-methyltetrahydropyran.

In some embodiments, the carrier liquid compound includes a ketone.Ketones may be advantageous for use in some embodiments due to theirrelatively large dipole moments, which may allow for relatively highionic conductivity in the electrolyte. In some embodiments, the carrierliquid includes an acyclic ketone such as, for example, 2-butanone,2-pentanone, 3-pentanone, or 3-methyl-2-butanone. The carrier liquid caninclude, in some cases, a cyclic ketone including cyclic ketones with5-membered rings (e.g., cyclopentanone, 2-methyl cyclopentanone, and3-methyl cyclopentanone) or 6-membered rings (e.g., cyclohexanone,2-methyl cyclohexanone, 3-methyl cyclohexanone, 4-methyl cyclohexanone).

The carrier liquid compound can contain 2 oxygen atoms, in someembodiments. For example, the carrier liquid can include a diether, adiketone, or an ester. In some embodiments, the carrier liquid caninclude an acyclic diether (e.g., 1,2-dimethoxyethane,1,2-diethoxyethane) an acyclic diketone (e.g., 2,3-butanedione,2,3-pentanedione, 2,3-hexanedione), or an acyclic ester (e.g., ethylacetate, ethyl propionate, methyl propionate). The carrier liquid caninclude a cyclic diether, in some embodiments. For example, the carrierliquid can include a cyclic diether including 5-membered rings (e.g.,1,3-dioxolane, 2-methyl-1,3-dioxolane, 4-methyl-1,3-dioxolane), or acyclic diether including 6-membered rings (e.g., 1,3-dioxane,2-methyl-1,3-dioxane, 4-methyl-1,3-dioxane, 1,4-dioxane,2-methyl-1,4-dioxane). The carrier liquid can include a cyclic diketone,in some instances. For example, the carrier liquid can include a cyclicdiketone including 5-membered rings (e.g., 1,2-cyclopentanedione,1,3-cyclopentanedione, and 1H-indene-1,3(2H)-dione), or a cyclic dietherincluding 6-membered rings (e.g., 1,2-cyclohexane dione,1,3-cyclohexanedione, and 1,4-cyclohexanedione). In some embodiments,the carrier liquid can include a cyclic ester. For example, the carrierliquid can include a cyclic ester including 5-membered rings (e.g.,gamma-butyro lactone, gamma-valero lactone), or a cyclic ester including6-membered rings (e.g., delta-valero lactone, delta-hexa lactone).

In some cases, carrier liquid compounds containing 3 oxygen atoms may beemployed. For example, the carrier liquid may include a triether. Insome cases, the carrier liquid may include an acyclic triether such as,for example, 1-methoxy-2-(2-methoxyethoxy)ethane, and1-ethoxy-2-(2-ethoxyethoxy)ethane, or trimethoxymethane. In some cases,the carrier liquid can include a cyclic triether. In some embodiments,the carrier liquid can include a cyclic triether with 5-membered rings(e.g., 2-methoxy-1,3-dioxolane) or a cyclic triether with 6-memberedrings (e.g., 1,3,5-trioxane, 2-methoxy-1,3-dioxane,2-methoxy-1,4-dioxane).

The carrier liquid compound includes, in some embodiments, a carbonate(e.g., unsaturated carbonates). The carbonates may, in some cases, forman SEI at a lower potential than liquid carbonates conventionally usedin commercial lithium batteries. In some instances, acyclic carbonatescan be used (e.g., methyl vinyl carbonate, methyl ethynyl carbonate,methyl phenyl carbonate, phenyl vinyl carbonate, ethynyl phenylcarbonate, divinyl carbonate, diethynyl carbonate, diphenyl carbonate).In some instances, cyclic carbonates can be used such as, for examplecyclic carbonates with 6-membered rings (e.g., 1,3-dioxan-2-one).

In some embodiments, the carrier liquid includes compounds that includea combination of one or more ethers, esters, and/or ketones. Suchstructures can be advantageous for use in some embodiments due to theirrelatively high dipole moments, allowing for high ionic conductivity inthe electrolyte. In some embodiments, the carrier liquid includes anether-ester (e.g., 2-methoxyethyl acetate), an ester-ketone (e.g.,3-acetyldihydro-2(3H)-furanone, 2-oxopropyl acetate), a diether-ketone(e.g., 2,5-dimethoxy-cyclopentanone, 2,6-dimethoxy-cyclohexanone), or ananhydride (e.g., acetic anhydride).

In some cases, the carrier liquid compound includes one nitrogen and oneoxygen atom such as an amide. Such compounds can be acyclic (e.g.,N,N-dimethyl formamide) or cyclic (e.g., 1-methyl-2-pyrrolidone,1-methyl-2-piperidone, 1-vinyl-2-pyrrolidone).

Compounds containing 1 nitrogen and 2 oxygen atoms can be used in thecarrier liquid, in some cases. For example,3-methyl-1,3-oxazolidin-2-one can be used as a carrier liquid, in somecases. 3-methyl-1,3-oxazolidin-2-one may be advantageous for use in someembodiments due to its relatively high dipole moment, which would allowfor high ionic conductivity in the electrolyte.

Compounds containing two nitrogen atoms and one oxygen atom can be usedin the carrier liquid, in some instances. For example, in someembodiments, the carrier liquid can include1,3-dimethyl-2-imidazolidinone, N,N,N′,N′-tetramethylurea, or1,3-dimethyltetrahydro-2(1H)-pyrimidinone. These compounds also includea relatively high dipole moment, which can provide advantages in someembodiments.

In some cases, the carrier liquid includes fluorinated or nitrilecompounds (e.g., fluorinated or nitrile derivatives of any of thecarrier liquid types mentioned herein). Such compounds may increase thestability of the fluid and allow for higher ionic conductivity of theelectrolytes. Examples of such fluorinated compounds include, but arenot limited to, 2,2-difluoro-1,3-dioxolane,2,2,5,5-tetrafluorocyclopentaone, 2,2-difluoro-gama-butyrolactone, and1-(trifluoromethyl)pyrrolidin-2-one. Examples of such nitrile compoundsinclude, but are not limited to, tetrahydrofuran-2-carbonitrile,1,3-dioxolane-2-carbonitrile, and 1,4-dioxane-2-carbonitrile.

In some cases, the carrier liquid includes sulfur containing compounds.In some cases, the carrier liquid can include a sulfoxide (e.g.,dimethyl sulfoxide, tetrahydrothiophene 1-oxide,1-(methylsulfonyl)ethylene), a sulfone (e.g., dimethyl sulfone, divinylsulfone, tetrahydrothiophene 1,1-dioxide), a sulfite (e.g.,1,3,2-dioxathiolane 2-oxide, dimethyl sulfite, 1,2-propyleneglycolsulfite), or a sulfate (e.g., dimethyl sulfate, 1,3,2-dioxathiolane2,2-dioxide). In some embodiments, the carrier liquid can include acompound with 1 sulfur and 3 oxygen atoms (e.g., methylmethanesulfonate, 1,2-oxathiolane 2,2-dioxide, 1,2-oxathiane2,2-dioxide, methyl trifluoromethanesulfonate).

The carrier liquid includes, in some embodiments, phosphorous containingcompounds such as, for example, phosphates (e.g., trimethyl phosphate)and phosphites (e.g., trimethyl phosphite). In some embodiments, thecarrier liquid can include 1 phosphorus and 3 oxygen atoms (e.g.,dimethyl methylphosphonate, dimethyl vinylphosphonate).

In some embodiments, the carrier liquid includes an ionic liquid. Theuse of ionic liquids may significantly reduce or eliminate SEIformation, in some cases. Exemplary anions suitable for use in the ionicliquid include, but are not limited to tetrafluoroborate,hexafluorophosphate, hexafluoroarsenoate, perchlorate,trifluoromethanesulfonate, bis(trifluoromethylsulfonyl)amide, andthiosaccharin anion. Suitable cations include, but are not limited to,ammonium, imidazolium, pyridinium, piperidinium or pyrrolidiniumderivatives. The ionic liquid can, in some embodiments, include acombination of any one of the above anions and any one of the abovecations.

The carrier liquid includes, in some cases, perfluorinated derivates ofany of the carrier liquid compounds mentioned herein. A perfluorinatedderivative is used to refer to compounds in which at least one hydrogenatom bonded to carbon atom is replaced by a fluorine atom. In somecases, at least half or substantially all of the hydrogen atoms bondedto a carbon atom are replaced with a fluorine atom. The presence of oneor more fluorine atoms in the carrier liquid compound may, in someembodiments, allow for enhanced control over the viscosity and/or dipolemoment of the molecule.

In some embodiments, the viscosity of the redox compositions undergoingflow can be within a very broad range, from about 1 centipoise (cP) toabout 1.5×10⁶ cP or from about 1 centipoise (cP) to about 10⁶ cP at theoperating temperature of the battery, which may be between about −50° C.and +50° C. In some embodiments, the viscosity of the electrodeundergoing flow is less than about 10⁵ cP. In other embodiments, theviscosity is between about 100 cP and 10⁵ cP. In those embodiments wherea semi-solid is used, the volume percentage of ion-storing solid phasesmay be between 5% and 70%, and the total solids percentage includingother solid phases such as conductive additives may be between 10% and75%. In some embodiments, the cell “stack” where electrochemicalreaction occurs operates at a higher temperature to decrease viscosityor increase reaction rate, while the storage tanks for the semi-solidmay be at a lower temperature.

In some embodiments, peristaltic pumps are used to introduce asolid-containing electroactive material into an electroactive zone, ormultiple electroactive zones in parallel. The complete volume (occupiedby the tubing, a slurry reservoir, and the active cells) of the slurrycan be discharged and recharged by slurry cycling. The active positiveelectrode and negative electrode slurries can be independently cycledthrough the cell by means of peristaltic pumps. The pump can provideindependent control of the flow rates of the positive electrode slurryand the negative electrode slurry. The independent control permits powerbalance to be adjusted to slurry conductivity and capacity properties.

In some embodiments, the peristaltic pump works by moving a roller alonga length of flexible tubing. This way the fluid inside the tubing nevercomes into contact with anything outside of the tubing. In a pump, adrive turns a shaft which is coupled to a pump head. The pump headsecures the tubing in place and also use the rotation of the shaft tomove a rolling head across the tubing to create a flow within the tube.Such pumps are often used in situations where the fluid beingtransferred needs to be isolated (as in blood transfusions and othermedical applications). Here the peristaltic pump can also be used totransfer viscous fluids and particle suspensions. In some embodiments, aclosed circuit of tubing is used to run the slurry in a cycle, withpower provided by the peristaltic pump. In some embodiments, the closedanolyte and catholyte systems may be connected to removable reservoirsto collect or supply anolyte and catholyte; thus enabling the activematerial to be recycled externally. The pump will require a source ofpower which may include that obtained from the cell. In someembodiments, the tubing may not be a closed cycle, in which caseremovable reservoirs for charged and of discharged anolytes andcatholytes can be employed; thus enabling the active material to berecycled externally. In some embodiments, one or more slurries arepumped through the redox cell at a rate permitting complete charge ordischarge during the residence time of the slurry in the cell, whereasin other embodiments one or more slurries are circulated repeatedlythrough the redox cell at a higher rate, and only partially charged ordischarged during the residence time in the cell. In some embodimentsthe pumping direction of one or more slurries is intermittently reversedto improve mixing of the slurries or to reduce clogging of passages inthe flow system.

While peristaltic pumps have been described in detail, it should beunderstood that other types of pumps can also be used to transport theflowable redox composition(s) described herein. For example, in someembodiments, a piston pump is used to transport one or more flowableredox compositions through the redox flow energy storage device. In someembodiments, an auger can be used to transport one or more flowableredox compositions.

The flowable redox compositions can include various additives to improvethe performance of the flowable redox cell. The liquid phase of thesemi-solid slurry in such instances would comprise a solvent, in whichis dissolved an electrolyte salt, and binders, thickeners, or otheradditives added to improve stability, reduce gas formation, improve SEIformation on the negative electrode particles, and the like. Examples ofsuch additives include vinylene carbonate (VC), vinylethylene carbonate(VEC), fluoroethylene carbonate (FEC), or alkyl cinnamates, to provide astable passivation layer on the anode or thin passivation layer on theoxide cathode; propane sultone (PS), propene sultone (PrS), or ethylenethiocarbonate as antigassing agents; biphenyl (BP), cyclohexylbenzene,or partially hydrogenated terphenyls, as gassing/safety/cathodepolymerization agents; or lithium bis(oxatlato)borate as an anodepassivation agent.

In some embodiments, the nonaqueous positive and negative electrodeflowable redox compositions are prevented from absorbing impurity waterand generating acid (such as HF in the case of LiPF₆ salt) byincorporating compounds that getter water into the active materialsuspension or into the storage tanks or other plumbing of the system.Optionally, the additives are basic oxides that neutralize the acid.Such compounds include but are not limited to silica gel, calciumsulfate (for example, the product known as Drierite), aluminum oxide andaluminum hydroxide.

In some embodiments, the colloid chemistry and rheology of thesemi-solid flow electrode is adjusted to produce a stable suspensionfrom which the solid particles settle only slowly or not at all, inorder to improve flowability of the semi-solid and to minimize anystirring or agitation needed to avoid settling of the active materialparticles. The stability of the electroactive material particlesuspension can be evaluated by monitoring a static slurry for evidenceof solid-liquid separation due to particle settling. As used herein, anelectroactive material particle suspension is referred to as “stable”when there is no observable particle settling in the suspension. In someembodiments, the electroactive material particle suspension is stablefor at least 5 days. Usually, the stability of the electroactivematerial particle suspension increases with decreased suspended particlesize. In some embodiments, the particle size of the electroactivematerial particle suspension is about less than 10 microns. In someembodiments, the particle size of the electroactive material particlesuspension is about less than 5 microns. In some embodiments, theparticle size of the electroactive material particle suspension is about2.5 microns. In some embodiments, conductive additives are added to theelectroactive material particle suspension to increase the conductivityof the suspension. Generally, higher volume fractions of conductiveadditives such as Ketjen carbon particles increase suspension stabilityand electronic conductivity, but excessive amount of conductiveadditives may also increase the viscosity of the suspension. In someembodiments, the flowable redox electrode composition includesthickeners or binders to reduce settling and improve suspensionstability. In some embodiments, the shear flow produced by the pumpsprovides additional stabilization of the suspension. In someembodiments, the flow rate is adjusted to eliminate the formation ofdendrites at the electrodes.

In some embodiments, the active material particles in the semi-solid areallowed to settle and are collected and stored separately, then re-mixedwith the liquid to form the flow electrode as needed.

In some embodiments, the rate of charge or discharge of the redox flowbattery is increased by increasing the instant amount of one or bothflow electrodes in electronic communication with the current collector.

In some embodiments, this is accomplished by making the semi-solidsuspension more electronically conductive, so that the reaction zone isincreased and extends into the flow electrode. In some embodiments, theconductivity of the semi-solid suspension is increased by the additionof a conductive material, including but not limited to metals, metalcarbides, metal nitrides, and forms of carbon including carbon black,graphitic carbon powder, carbon fibers, carbon microfibers, vapor-growncarbon fibers (VGCF), and fullerenes including “buckyballs”, carbonnanotubes (CNTs), multiwall carbon nanotubes (MWNTs), single wall carbonnanotubes (SWNTs), graphene sheets or aggregates of graphene sheets, andmaterials comprising fullerenic fragments that are not predominantly aclosed shell or tube of the graphene sheet. In some embodiments, nanorodor nanowire or highly expected particulates of active materials orconductive additives can be included in the electrode suspensions toimprove ion storage capacity or power or both. As an example, carbonnanofilters such as VGCF (vapor growth carbon fibers), multiwall carbonnanotubes (MWNTs) or single-walled carbon nanotubes (SWNTs), may be usedin the suspension to improve electronic conductivity, or optionally tostore the working ion.

In some embodiments, the conductivity of the semi-solid ion-storingmaterial is increased by coating the solid of the semi-solid ion-storingmaterial with a conductive coating material which has higher electronconductivity than the solid. Non-limiting examples of conductive-coatingmaterial include carbon, a metal, metal carbide, metal nitride, metaloxide, or conductive polymer. In some embodiments, the solid of thesemi-solid ion-storing material is coated with metal that is redox-inertat the operating conditions of the redox energy storage device. In someembodiments, the solid of the semi-solid ion-storing material is coatedwith copper to increase the conductivity of the storage materialparticle, to increase the net conductivity of the semi-solid, and/or tofacilitate charge transfer between energy storage particles andconductive additives. In some embodiments, the storage material particleis coated with, about 1.5% by weight, metallic copper. In someembodiments, the storage material particle is coated with, about 3.0% byweight, metallic copper. In some embodiments, the storage materialparticle is coated with, about 8.5% by weight, metallic copper. In someembodiments, the storage material particle is coated with, about 10.0%by weight, metallic copper. In some embodiments, the storage materialparticle is coated with, about 15.0% by weight, metallic copper. In someembodiments, the storage material particle is coated with, about 20.0%by weight, metallic copper. In general, the cycling performance of theflowable redox electrode increases with the increases of the weightpercentages of the conductive coating material. In general, the capacityof the flowable redox electrode also increases with the increases of theweight percentages of the conductive coating material.

In some embodiments, the rate of charge or discharge of the redox flowbattery is increased by adjusting the interparticle interactions orcolloid chemistry of the semi-solid to increase particle contact and theformation of percolating networks of the ion-storage material particles.In some embodiments, the percolating networks are formed in the vicinityof the current collectors. In some embodiments, the semi-solid isshear-thinning so that it flows more easily where desired. In someembodiments, the semi-solid is shear thickening, for example so that itforms percolating networks at high shear rates such as those encounteredin the vicinity of the current collector.

The energy density of nonaqueous batteries using the flowable electrodeactive materials according to one or more embodiments compares favorablyto conventional redox anolyte and catholyte batteries. Redox anolytesand catholytes, for example those based on vanadium ions in solution,typically have a molar concentration of the vanadium ions of between 1and 8 molar, the higher concentrations occurring when high acidconcentrations are used. One may compare the energy density of asemi-solid slurry based on known lithium ion battery positive andnegative electrode compounds to these values. The liquid phase of thesemi-solid slurry in such instances would comprise a solvent, includingbut not limited to an alkyl carbonate or mixture of alkyl carbonates, inwhich is dissolved a lithium salt, including but not limited to LiPF₆,and binders, thickeners, or other additives added to improve stability,reduce gas formation, improve SEI formation on the negative electrodeparticles, and the like.

In a non-aqueous semi-solid redox flow cell, one useful positiveelectrode flowable redox composition is a suspension of lithiumtransition metal olivine particles in the liquid discussed above. Sucholivines include LiMPO₄ where M comprises a first row transition metals,or solid solutions, doped or modified compositions, or nonstoichiometricor disordered forms of such olivines. Taking the compound LiFePO₄ forillustrative example, the density of olivine LiFePO₄ is 3.6 g/cm³ andits formula weight is 157.77 g/mole. The concentration of Fe per literof the solid olivine is therefore: (3.6/157.77)×1000 cm³/liter=22.82molar. Even if present in a suspension diluted substantially by liquid,the molar concentration far exceeds that of typical redox electrolytes.For example, a 50% solids slurry has 11.41M concentration, exceedingeven highly concentrated vanadium flow battery electrolytes, and this isachieved without any acid additions.

In some embodiments, a positive electrode flowable redox composition inwhich the electrochemically active solid compound forming the particlesis LiCoO₂, the density is 5.01 g/cm³ and the formula weight is 97.874g/mole. The concentration of Co per liter is: (5.01/97.874)×1000cm³/liter=51.19 molar. The energy density of such semi-solid slurries isclearly a factor of several higher than that possible with conventionalliquid catholyte or anolyte solutions.

In some embodiments, a suspension of graphite in the liquid, which mayserve as a negative electrode flowable redox composition, is used. Inoperation, graphite (or other hard and soft carbons) can intercalatelithium. In graphite the maximum concentration is about LiC₆. Sincegraphite has a density of about 2.2 g/cm³, and the formula weight ofLiC₆ is 102.94 g/mole, the concentration of Li per liter of LiC₆ is:(2.2/102.94)×1000=21.37 molar. This is again much higher thanconventional redox flow battery anolytes.

Furthermore, the nonaqueous batteries can have cell working voltagesthat are more than twice as high as some aqueous batteries, where thevoltage can be limited to 1.2-1.5V due to the limitation of waterhydrolysis at higher voltage. By contrast, use of LiFePO₄ with graphitein a semi-solid redox flow cell provides 3.3V average voltage, andLiCoO₂ with graphite provides 3.7V average voltage. Since the energy ofany battery is proportional to voltage, the batteries using solidsuspension or condensed ion-supporting liquid redox flow compositionshave a further improvement in energy over conventional solution-basedredox flow cells.

Thus a non-aqueous semi-solid redox flow cell can provide the benefitsof both redox flow batteries and conventional lithium ion batteries byproviding for a higher cell voltage and for flow battery electrodes thatare much more energy dense than redox flow batteries by not beinglimited to soluble metals, but rather, comprising a suspension of solidor liquid electrode-active materials, or in the case of dense liquidreactants such as liquid metals or other liquid compounds, the flowbattery electrolyte may comprise a significant fraction or even amajority of the liquid reactant itself. Unlike a conventional primary orsecondary battery, the total capacity or stored energy may be increasedby simply increasing the size of the reservoirs holding the reactants,without increasing the amount of other components such as the separator,current collector foils, packaging, and the like. Unlike a fuel cell,such a semi-solid redox flow battery is rechargeable.

Amongst many applications, the semi-solid and condensed ion-supportingliquid redox flow batteries can be used to power a plug-in hybrid (PHEV)or all-electric vehicle (EV). Currently, for markets where the dailydriving distance is long, such as the U.S. where the median dailydriving distance is 33 miles, PHEVs are an attractive solution becausewith daily charging a battery that supplies 40 miles of electric range(PHEV40) is practical. For a car weighing about 3000 lb this requires abattery of approximately 15 kWh of energy and about 100 kW power, whichis a battery of manageable size, weight, and cost.

However, an EV of the same size for the same driving pattern generallywill require longer range, such as a 200 mile driving distance betweenrecharges, or 75 kWh, in order to provide an adequate reserve of energyand security to the user. Higher specific energy batteries are needed tomeet the size, weight and cost metrics that will enable widespread useof EVs. The semi-solid and condensed ion-supporting liquid redox flowbatteries can enable practical low cost battery solutions for suchapplications. The theoretical energy density of the LiCoO₂/carbon coupleis 380.4 Wh/kg. However, high power and high energy lithium ionbatteries based on such chemistry provide only about 100-175 Wh/kg atthe cell level, due to the dilution effects of inactive materials.Providing a 200 mile range, which is equivalent to providing 75 kWh ofenergy, requires 750-430 kg of current advanced lithium ion cells.Additional mass is also required for other components of the batterysystem such as packaging, cooling systems, the battery managementsystem, and the like.

Considering the use of conventional lithium ion batteries in EVs, it isknown that specific energy is more limiting than power. That is, abattery with sufficient energy for the desired driving range willtypically have more than enough power. Thus the battery system includeswasted mass and volume that provides unneeded power. The semi-solid orcondensed ion-supporting liquid redox flow battery can have a smallerpower-generating portion (or stack) that is sized to provide thenecessary power, while the remaining, larger fraction of the total masscan be devoted to the high energy density positive and negativeelectrode redox flow compositions and their storage system. The mass ofthe power-generating stack is determined by considering how much stackis needed to provide the approximately 100 kW needed to operate the car.Lithium ion batteries are currently available that have specific powerof about 1000-4000 W/kg. The power generated per unit area of separatorin such a battery and in the stacks of the flowable redox cell issimilar. Therefore, to provide 100 kW of power, about 25-100 kg of stackis needed.

The remainder of the battery mass may come predominantly from thepositive and negative electrode flowable redox compositions. As thetheoretical energy density for the LiCoO₂/carbon couple is 380.4 Wh/kg,the total amount of active material required to provide 75 kWh of energyis only 197 kg. In flow batteries the active material is by far thelargest mass fraction of the positive and negative electrode flowableredox compositions, the remainder coming from additives and liquidelectrolyte phase, which has lower density than the ion storagecompounds. The mass of the positive and negative electrode flowableredox compositions needed to supply the 75 kWh of energy is only about200 kg.

Thus, including both the stack mass (25-100 kg) and the positive andnegative electrode flowable redox composition mass (200 kg), asemi-solid redox flow battery to supply a 200 mile range may weigh 225to 300 kg mass, much less than the mass (and volume) of advanced lithiumion batteries providing the same range. The specific energy of such asystem is 75 kWh divided by the battery mass, or 333 to 250 Wh/kg, abouttwice that of current lithium cells. As the total energy of the systemincreases, the specific energy approaches the theoretical value of 380.4Wh/kg since the stack mass is a diminishing fraction of the total. Inthis respect the rechargeable lithium flow battery has different scalingbehavior than conventional lithium ion cells, where the energy densityis less than 50% of the theoretical value regardless of system size, dueto the need for a large percentage of inactive materials in order tohave a functioning battery.

Thus in one set of embodiments, a rechargeable lithium ion flow batteryis provided. In some embodiments, such a battery has a relatively highspecific energy at a relatively small total energy for the system, forexample a specific energy of more than about 150 Wh/kg at a total energyof less than about 50 kWh, or more than about 200 Wh/kg at total energyless than about 100 kWh, or more than about 250 Wh/kg at total energyless than about 300 kWh.

In another set of embodiments, a redox flow device uses one or morereference electrode during operation to determine the absolute potentialat the positive and negative current collectors, the potentials beingused in a feedback loop to determine the appropriate delivery rate ofpositive and negative electrode flowable redox compositions. Forexample, if the cathodic reaction is completing faster than the anodicreaction, the cell will be “cathode-starved” and greater polarizationwill occur at the positive electrode. In such an instance, detection ofthe cathode potential will indicate such a condition or impendingcondition, and the rate of delivery of positive electrode flowable redoxcomposition can be increased. If the redox flow cell is being used athigh power, and both cathode and anode reactions are completing andresulting in a fully discharged or charged state at the instant flowrates, this too can be detected using the current collector potentials,and the rates of both positive and negative electrode flowable redoxcompositions are increased so as to “match” the desired current rate ofthe cell.

More than one reference electrode may be used in order to determine thepositional variation in utilization and completeness of electrochemicalreaction within the flow battery. Consider for example a planar stackwherein the positive and negative electrode flowable redox compositionsflow parallel to the separator and electrodes, entering the stack at oneend and exiting at the other. Since the cathode-active and anode-activematerials can begin to charge or discharge as soon as they are inelectrical communication, the extent of reaction can differ at theentrance and the exit to the stack. By placing reference electrodes atmore than one position within the stack and within the cell, thenear-instantaneous state of the cell with respect to state of charge ordischarge and local polarization can be determined. The operatingefficiency, power and utilization of the cell can be optimized by takinginto account the voltage inputs from the reference electrodes andaltering operating parameters such as total or relative flow rate ofcatholyte and anolyte.

The reference electrodes may also be placed elsewhere within the flowdevice system. For example, having reference electrodes in the positiveand negative electrode flowable redox composition storage tanks, orhaving a separate electrochemical cell within the storage tanks, thestate of charge and discharge of the positive and negative electrodeflowable redox compositions in the tank can be monitored. This also canbe used as input to determine the flow rate of the semi-solidsuspensions when operating the battery in order to provide necessarypower and energy. The position of the reference electrode permits thedetermination of the local voltage in either the anolyte, catholyte, orseparator. Multiple reference electrodes permit the spatial distributionof voltage to be determined. The operating conditions of the cells,which may include flow rates, can be adjusted to optimize power densityvia changes in the distribution of voltage.

In some embodiments, the semi-solid redox flow cell is a nonaqueouslithium rechargeable cell and uses as the reference electrode a lithiumstorage compound that is lithiated so as to produce a constant potential(constant lithium chemical potential) over a range of lithiumconcentrations. In some embodiments the lithium-active material in thereference electrode is lithium titanate spinel or lithium vanadium oxideor a lithium transition metal phosphate including but not limited to alithium transition metal olivine of general formula Li_(x)M_(y)PO₄ whereM comprises a first row transition metal. In some embodiments thecompound is LiFePO₄ olivine or LiMnPO₄ olivine or mixtures or solidsolutions of the two.

In some cases, a discrete in-line sensor, which can be separate from theflow device system (and therefore, removable from the system), may beused during operation of the system. The in-line sensor may contain areference electrode that can be used to determine an absolute potentialat a location within the system such as, for example, a flowable redoxcomposition storage tank or a conduit that transports a flowable redoxcomposition within the system. The use of one or more discrete in-linesensors that can be removed from the redox flow energy storage systemmay reduce the amount of time needed to replace the sensors, thusreducing system downtime. In addition, the data obtained from a discretein-line sensor may be compared to the data obtained from an integratedreference electrode, which may be useful in checking the accuracy of oneor more integrated reference electrodes.

In some embodiments, the discrete in-line sensor is positionedexternally to the electroactive zones of the flow cell and/or externallyto the source(s) of flowable redox active material (e.g., outside thestorage tank(s) containing the flowable redox active material). Thediscrete in-line sensor is, in some embodiments, external to a conduitused to directly fluidically connect the source of flowable redox activematerial to an electroactive zone, as described in more detail below.

In one set of embodiments, an in-line sensor can be used to determinethe condition of a positive and/or negative flowable redox composition(e.g., catholyte and/or anolyte), including but not limited to itsstate-of-charge, electronic or ionic conductivity, state of aggregation,viscosity and state of health by measuring the time dependence of suchcharacteristics. In some embodiments, the in-line sensor can determinecertain properties of a positive and/or negative flowable redoxcomposition by taking measurements from a flowable redox compositionthat is undergoing flow. In-line sensors may, in some instances,determine properties of a flowable redox composition by sampling atleast a portion of the flowable redox composition that is diverted fromthe main flow channels through said sensor. For example, the in-linesensor may divert a portion of a flowable redox composition from theredox flow energy storage device to an independent conduit to determineone or more properties of the flowable redox composition. Measurementsthat can be performed by the sensor include, but are not limited to thefollowing properties of the flowable redox composition: itselectrochemical potential with respect to a reference electrodecontained within said sensor; its DC conductivity, measured acrossconductive electrodes contacting the flowable redox composition; itsviscosity; its AC conductivity, from which transport and dielectricproperties may be obtained using impedance spectroscopy methods known tothose of ordinary skill in the art; and/or its magnetic properties.These properties may be measured as a function of time, temperature,flow rate, and/or amplitude and/or frequency of an applied potential orfield. For example, the electrochemical potential of a flowable redoxcomposition may be used to determine its state-of-charge duringoperation of the flow cell, or to provide information on degradation ofthe flowable redox composition (e.g., a component within the flowableredox composition). The DC or AC conductivity may be used to determinethe state of percolation of a conductive solid phase in a semi-solidsuspension flowable redox composition, and/or the rate of SEIaccumulation. The variation of these quantities with flow rate may beused as feedback to determine operating conditions under which enhancedcell performance is observed. For example, the C-rate at which the flowcell may be charged or discharged may be maximized at a certain flowrate of the positive and/or negative flowable redox composition.

A flowable redox composition can be mixed within the redox flow energystorage device via a variety of mechanisms. Mixing the flowable redoxcomposition can allow for the concentration of a redox species to beincreased, for example, near a current collector and/or a ion-permeablemedium within the redox flow energy storage device, thereby enhancingthe performance of the device. In one set of embodiments, heat (e.g.,heat generated by an electrochemical reaction within the flow cell, heatgenerated by an external apparatus such as a resistive heater, etc.) canbe present within or can be applied to a portion of the redox flowenergy storage device to selectively heat one portion of the devicerelative to another. This can increase the temperature of at least aportion of the anodic and/or cathodic fluids. For example, in the set ofembodiments illustrated in FIG. 1H, region 750 of the energy storagedevice is hotter than region 752, producing. Natural convection can beinduced via decreased density within the active regions of the flowcell, assisting in the pumping of the positive and/or negative flowableredox composition through the flow cell with reduced energy consumption.In the set of embodiments illustrated in FIG. 1H, each of the positiveand negative flowable redox compositions abuts an electricallyinsulating, ion-permeable medium 614 (e.g., a membrane). The inset ofFIG. 1H (located in the upper-left-hand corner) includes a schematicillustration of the device without the positive and negative flowableredox compositions positioned within the electroactive zones.

In some embodiments, a mixing fluid can be used to increase the amountof mixing within a flowable redox composition within the redox flowenergy storage device. In some embodiments, the redox flow energystorage device includes a source of mixing fluid in fluid communicationwith and/or located within a volume in which the flowable redoxcomposition is disposed (e.g., an electroactive region). In most cases,the mixing fluid is immiscible with the flowable redox composition. Asused herein, two fluids are “immiscible,” or not miscible, with eachother when one is not soluble in the other to a level of at least 10% byweight at the temperature and under the conditions at which the redoxflow energy storage device is operated.

The mixing fluid can be of any suitable type, including liquids andgases. In some embodiments, the mixing fluid is not substantiallychemically reactive with the flowable redox composition. As used herein,a component is “not substantially chemically reactive” with anothercomponent if, when the two components are contacted with each other, achemical reaction does not proceed over the time scale of use of thedevice of the invention.

In some embodiments, the source of the mixing fluid can comprise avolume external to the redox flow energy storage device. For example, insome cases, the redox flow energy storage device comprises a mixingfluid such as a gas (e.g., an inert gas) that originates from a sourceindependent of the redox flow energy storage device, and is transported(e.g., injected via a conduit) into a positive and/or negative flowableredox composition. One such set of embodiments is illustrated in FIG.1J. In this set of embodiments, fluid bubbles 910 are injected into theelectroactive regions of the redox flow energy storage device viachannels 912, which are in fluid communication with electroactiveregions 115 and 125 of the redox flow energy storage device. In additionto promoting mixing, the injected fluid can assist in the pumping of thefluids through the flow cell. The bubbles can rise due to buoyantforces, and induce fluid flow in each fluid. In the set of embodimentsillustrated in FIG. 1J, each flowing fluid abuts an electricallyinsulating, ion-permeable medium (e.g., a membrane). The inset of FIG.1J (located in the upper-left-hand corner) includes a schematicillustration of the device without the positive and negative flowableredox compositions positioned within the electroactive zones.

In some embodiments, the source of the mixing fluid can be a reactantwithin the redox flow energy storage device. For example, the mixingfluid (e.g., gas bubbles) can be produced, in some embodiments, as aside product of a reaction in the redox flow energy storage device. FIG.1K includes a schematic illustration of one such set of embodiments. InFIG. 1K, fluid bubbles 910 are produced as a side product of anelectrochemical reaction. The gas can rise due to buoyant forces,thereby assisting in the pumping of the catholyte and anolyte, andreducing the amount of energy needed to transport fluids within thecell. The mixing fluid bubbles can be created in the cathodic fluid, theanodic fluid, or in each of the positive and negative flowable redoxcomposition (as illustrated in FIG. 1K). In one set of embodiments, oneor both of the geometry and/or surface chemistry of the flow-cellvessels can be designed to control the location and rate of bubblenucleation, for example by providing indentations or cavities where theenergy barrier to heterogeneous nucleation of a bubble is lowered. Inthe set of embodiments illustrated in FIG. 1K, each flowing electrodefluid abuts an electrically insulating, ion-permeable medium (e.g., amembrane). The inset of FIG. 1K (located in the upper-left-hand corner)includes a schematic illustration of the device without the positive andnegative flowable redox compositions positioned within the electroactivezones.

The redox flow energy device can include, in some embodiments, a movablesurface in contact with the flowable redox composition within the redoxflow energy device. Generally, a surface is a “movable surface” if it iscapable of being moved relative to other surfaces within the redox flowdevice. For example, in some embodiments, the movable surface can bemovable relative to at least one current collector within the redox flowdevice (e.g., the movable surface can be part of a first currentcollector that is movable relative to a second current collector in thedevice). In some cases, the movable surface can be movable relative toan ion-permeable medium (e.g., a membrane) within the redox flow energystorage device. In some embodiments, at least a portion of the movablesurface can be disposed within and/or be in contact with anelectroactive zone within the redox flow energy storage device. Forexample, in the set of embodiments illustrated in FIG. 1C, at least aportion of internal surface 591 of negative electrode current collector520 and/or at least a portion of internal surface 592 of positiveelectrode current collector 510 can be movable.

The movable surface can be constructed and arranged to at leastpartially direct the flow of the flowable redox composition through theredox flow energy device. This can be achieved, for example, byincluding one or more protrusions on the movable surface, and moving thesurface such that the flowable redox composition is transported throughthe redox flow energy device.

As one particular example, in some embodiments, the flow cell cancomprise one or both of an internal auger and an external auger, whichmay be used, for example, to transport an anodic and/or a cathodicfluid. Such an arrangement was briefly described in relation to the setof embodiments illustrated in FIGS. 1B-1C. FIGS. 1F-1G include schematicillustrations of a set of embodiments in which internal auger 610 and anexternal auger 612 comprising threaded, movable surfaces are employed.One or both of the internal and external augers can be rotated relativeto a fixed ion-permeable medium 614 between the augers and/or eachother, for example in the direction of arrow 615. In this way, theaugers can transport fluid through the flow cell. For example, in oneset of embodiments, a positive flowable redox composition can betransported into the flow cell via arrows 616 and out of the flow cellvia arrows 617. In addition, the negative flowable redox composition canbe transported into the flow cell via arrows 618 and out of the flowcell via arrows 619. Embodiments in which augers are employed may beadvantageous in transporting or inducing mixing within fluids withrelatively high viscosities. In the set of embodiments, illustrated inFIGS. 1F-1G, the anodic and cathodic fluids can be transported throughconcentric cylindrical shells, each of which can abut an electricallyinsulating, ion-permeable medium 614 (e.g., a membrane). In someembodiments, the augers can be electronically conductive, which canallow for the augers to act as current collectors and/or electronicallycommunicate with separate current collectors in the flow cell.

The moving surface can be of a variety of other forms. For example, insome instances, the flow cell can comprise a moving surface that is partof a track drive that can be used to transport a fluid within anelectroactive region. The track drive can comprise a belt arrangedaround one or more rotatable axles, and can be at least partiallydisposed within an electroactive zone within the redox flow cell. Thetrack can be constructed and arranged such that at least a portion ofthe surface exposed to the redox flow material (which can be disposedwithin an electroactive zone within the redox flow cell) comprisesprotrusions that direct the flow of fluid. For example, FIG. 1L includesa schematic illustration of a flow cell comprising two track drives 710.The track drives in FIG. 1L can transport an anodic and a cathodic fluidindependently. The belts 712 on the track drives (which include movingsurfaces 714) include protrusions 716 that enhance the movement andmixing of fluid adjacent the drives. While the track drives illustratedin FIG. 1L include linear protrusions, any suitable protrusion geometrycan be employed. The fluids can be transported through a thin regionwith an approximately rectangular cross section, in some embodiments. Aswith the auger embodiments described above, the use of one or more trackdrives can be advantageous for transporting and/or mixing of fluids withrelatively high viscosities. In the set of embodiments illustrated inFIG. 1L, each flowing fluid abuts an electrically insulating,ion-permeable medium 614 (e.g., a membrane). In some embodiments, one orboth of the track drives can be electronically conductive, which canallow for the track drive(s) to act as a current collector and/or allowfor the track drive(s) to electronically communicate with separatecurrent collectors in the flow cell. The inset of FIG. 1L (located inthe upper-left-hand corner) includes a schematic illustration of thedevice without the positive and negative flowable redox compositionspositioned within the electroactive zones.

The flow cell can comprise, in some cases, a moving surface associatedwith a rotatable shaft that propels and/or impels an anodic and/or acathodic fluid through the flow cell. For example, FIG. 1M includes aschematic illustration of a flow cell that comprises multiple rotatableshafts 810. The external surfaces of the rotatable shafts can be movedby rotating the propellers about their longitudinal axes. The movablesurfaces of the rotatable shafts include a plurality of protrusions 812that help transport the fluid through the redox flow energy storagedevice. In this set of embodiments, each of rotatable shafts 810 impelsan anodic and a cathodic fluid independently through the active regionof the flow cell where electrochemical charging and discharging takesplace. The fluids can flow through a thin region with an approximatelyrectangular cross section. In the set of embodiments illustrated in FIG.1M, each flowing fluid abuts an electrically insulating, ion-permeablemedium 614 (e.g., a membrane).

Example 1 Semi-Solid Lithium Redox Flow Battery

An exemplary redox flow cell 200 for a lithium system is shown in FIG.2. In this example, the membrane 210 is a microporous membrane such as apolymer separator film (e.g., Celgard™ 2400) that prevents cathodeparticles 220 and anode particles 230 from crossing the membrane, or isa solid nonporous film of a lithium ion conductor. The negative andpositive electrode current collectors 240, 250 are made of copper andaluminum, respectively. The negative electrode composition includes agraphite or hard carbon suspension. The positive electrode compositionincludes LiCoO₂ or LiFePO₄ as the redox active component. Carbonparticulates are optionally added to the cathode or anode suspensions toimprove the electronic conductivity of the suspensions. The solvent inwhich the positive and negative active material particles are suspendedis an alkyl carbonate mixture and includes a dissolved lithium salt suchas LiPF₆. The positive electrode composition is stored in positiveelectrode storage tank 260, and is pumped into the electroactive zoneusing pump 265. The negative electrode composition is stored in negativeelectrode storage tank 270, and is pumped into the electroactive zoneusing pump 275. For the carbon and the LiCoO₂, the electrochemicalreactions that occur in the cell are as follows:Charge: xLi+6xC→xLiC₆ LiCoO₂ →xLi⁺+Li_(1-x)CoO₂Discharge: xLiC₆ →xLi+6xC xLi⁺+Li_(1-x)CoO₂→LiCoO₂

Example 2 Semi-Solid Nickel Metal Hydride Redox Flow Battery

An exemplary redox flow cell for a nickel system is shown in FIG. 3. Inthis example, the membrane 310 is a microporous electrolyte-permeablemembrane that prevents cathode particles 320 and anode particles 330from crossing the membrane, or is a solid nonporous film of a proton ionconductor, such as Nafion. The negative and positive electrode currentcollectors 340, 350 are both made of carbon. The negative electrodecomposition includes a suspension of a hydrogen absorbing metal, M. Thepositive electrode composition includes NiOOH as the redox activecomponent. Carbon particulates are optionally added to the cathode oranode suspensions to improve the electronic conductivity of thesuspensions. The solvent in which the positive and negative activematerial particles are suspended is an aqueous solution containing ahydroxyl generating salt such as KOH. The positive electrode compositionis stored in positive electrode storage tank 360, and is pumped into theelectroactive zone using pump 365. The negative electrode composition isstored in negative electrode storage tank 370, and is pumped into theelectroactive zone using pump 375. The electrochemical reactions thatoccur in the cell upon discharge are as follows (the reactions uponcharging being the reverse of these):Discharge: xM+yH₂O+ye ⁻→M_(x)H_(y) +yOH⁻ Ni(OH)₂+OH⁻→NiOOH+H₂O+e ⁻

Example 3 Reference Electrode Monitored Redox Flow Battery

An exemplary redox flow battery using a reference electrode to optimizecell performance is shown in FIG. 4. The cell includes two membranes410, 415. Reference electrodes 420, 425, 430 are positioned between thetwo membranes 410, 415 on a face opposite that of the electroactivezones 440, 445 where positive electrode redox flow composition 442 andnegative electrode redox flow composition 447 flow, respectively. Thecell also includes negative and positive current collectors 450, 460,respectively.

The potential at each reference electrode 420, 425 and 430 can bedetermined and are assigned a value of φ₁, φ₂ and φ₃, respectively. Thepotentials at the working electrodes (current collectors) 450, 460 canalso be determined and are assigned a value of W₁ and W₂, respectively.The potential differences of the cell components can be measured asfollows:(W ₁ −W ₂)=cell voltage(W ₂−φ₃)=potential at cathode(W ₁−φ₃)=potential at anodeφ₃−φ₂) or φ₂−φ₁)=extent of reaction as redox compositions flow alongstack.

In this example, three reference electrodes are used within the powergenerating stack (electroactive zone) in order to determine whether theflow rates of the positive and negative electrode redox flowcompositions are at a suitable rate to obtain a desired power. Forexample, if the flow rate is too slow during discharge, the positive andnegative electrode redox flow compositions fully discharge as the enterthe stack and over most of their residence time in the stack there isnot a high chemical potential difference for lithium. A higher flow rateallows greater power to be obtained. However, if the flow rate is toohigh, the active materials may not be able to fully charge or dischargeduring their residence time in the stack. In this instance the flow rateof the slurries may be slowed to obtain greater discharge energy, or oneor more slurries may be recirculated to obtain more complete discharge.In the instance of charging, too high a flow rate prevents the materialsfrom fully charging during a single pass, and the stored energy is lessthan the system is capable of, in which case the slurry flow rate may bedecreased, or recirculation used, to obtain more complete charging ofthe active materials available.

Example 4 Preparing Partially Delithiated, Jet-Milled Lithium CobaltOxide

Lithium cobalt oxide powder was jet-milled at 15,000 RPM to produceparticles with an average diameter of 2.5 microns. A 20 g sample ofjet-milled lithium cobalt oxide was chemically delithiated by reactingwith 2.5 g of nitronium tetrafluoroborate in acetonitrile over 24 hours.The delithiated Li_(1-x)CoO₂, having also a higher electronicconductivity by virtue of being partially delithiated, is used as theactive material in a cathode semi-solid suspension.

Example 5 Preparing a Copper Plated Graphite Powder

Commercial grade mesocarbon microbead (MCMB 6-28) graphitic anode powderwas partially coated with, 3.1% by weight, metallic copper via anelectroless plating reaction. MCMB (87.5 g) was stirred successively inthe four aqueous solutions listed in Table 1. Between each step, thepowder was collected by filtering and washed with reagent grade water.In the final solution, a concentrated solution of sodium hydroxide wasadded to maintain a pH of 12. Increasing the concentrations of thespecies in solution 4 would yield more copper rich powders. Powders withweight fractions 1.6%, 3.1%, 8.6%, 9.7%, 15%, and 21.4% copper werecharacterized by preparing slurries as described in Example 7, andtesting the slurries as described in Example 8. The cycling performanceincreased and capacity increased with copper plating weight percents asillustrated in FIG. 5.

TABLE 1 Four aqueous solutions used to treat MCMB. Solution ChemicalConcentration (M) 1 (1 hr) Nitric Acid 4.0 2 (2 hr) Stannous Chloride0.10 Hydrochloric Acid 0.10 3 (2 hr) Palladium Chloride 0.0058Hydrochloric Acid 0.10 4 (0.5 hr) Copper Sulfate 0.020 EDTA 0.050Formaldehyde 0.10 Sodium Sulfate 0.075 Sodium Formate 0.15 PolyethyleneGlycol 0.03 Sodium Hydroxide Maintain at pH 12

Example 6 Preparing a Cathode Slurry

A suspension containing 25% volume fraction of delithiated, jet-milledlithium cobalt oxide, 0.8% volume fraction of Ketjen Black, and 74.2%volume fraction of a standard lithium ion battery electrolyte wassynthesized. A stable cathode suspension was prepared by mixing 8.9 g ofdelithiated, jet-milled lithium cobalt oxide with 0.116 g of KetjenBlack carbon filler. The mixed powder was suspended in 5 mL ofelectrolyte and the suspension was sonicated for 20 minutes. Such asuspension was stable (i.e., there was no observable particle settling)for at least 5 days. The conductivity of such a suspension was measuredto be 0.022 S/cm in an AC impedance spectroscopy measurement. Suchslurries were tested in static and flowing cells as described in laterExamples. Experimentation with the relative proportions of theconstituents of the slurries showed that higher volume fractions oflithium cobalt oxide, which increase the storage capacity of thesuspension, can be made. Increasing the volume fraction of solids in thesuspension also increased the viscosity of the semi-solid suspensions.Higher volume fractions of Ketjen carbon particles increased suspensionstability and electronic conductivity, but also the slurry viscosity.Straightforward experimentation was used to determine volume fractionsof lithium cobalt oxide and Ketjen carbon that produce slurries ofsuitable viscosity for device operation.

Example 7 Preparing an Anode Slurry

A suspension containing 40% volume fraction of graphite in 60% volumefraction of a standard lithium ion battery electrolyte was synthesizedby mixing 2.88 g of copper plated graphite (3.1 wt % copper) with 2.0 mLof electrolyte. The mixture was sonicated for 20 minutes. Theconductivity of the slurry was 0.025 S/cm. Higher copper loadings on thegraphite was observed to increase the slurries' viscosity.

Example 8 Static Half Cell Tests on Cathode and Anode Slurries

Semi-solid suspension samples, as described in Examples 6 and 7, werecharged and discharged electrochemically against a lithium metalelectrode in an electrochemical cell where the suspension was static.The cathode or anode slurry was placed in a metallic well which alsoacted as the current collector. The well and current collectors weremachined from aluminum and copper for the cathode and anode,respectively. The wells holding the slurries had cylindrical shape 6.3mm in diameter and depths ranging from 250-800 μm. A Celgard 2500separator film separated the slurry from a lithium metal counterelectrode, and an excess of electrolyte was added to the gaps in thecell to ensure that the electrochemically tested materials remainedwetted with electrolyte. Testing was conducted in an argon-filledglovebox. A representative plot of voltage as a function of chargingcapacity for the cathode slurry half-cell is shown in FIG. 6. Arepresentative plot of the cathode discharge capacity vs. cycle numberis shown in FIG. 9. A representative plot of voltage as a function ofcharging capacity for the anode slurry half-cell is shown in FIG. 7.Both anode and cathode behaved electrochemically in a manner similar totheir solid (unsuspended) counterparts. Example capacity measurementsare shown in Table 2.

TABLE 2 Example capacity measurements. Specific Capacity in SpecificVolumetric mAh per gram Capacity in Capacity in of MCMB mAh per gram mAhper mL of Slurry Material or LiCoO₂ of Slurry Slurry MCMB with 0 wt % 9651 85 deposited Cu,¹ 40 vol % anode powder in electrolyte MCMB with 3.1wt % 344 179 300 Cu,² 40 vol % anode powder in electrolyte MCMB with 15wt % 252 123 219 Cu¹ 40 vol % anode powder in electrolyte MCMB with 21.4wt 420 190 354 % Cu,³ 40 vol % anode powder in electrolyte 26 vol %LiCoO₂, 0.8 97 56 127 vol % Ketjen Carbon Black in electrolyte⁴¹Capacity calculated from the 2^(nd) cycle discharge in a C/20galvanostatic cycling experiment between 0.01 V and 0.6 V versus Limetal; ²Capacity calculated from the 2^(nd) cycle discharge in a C/20CCCV charge, C/20 galvanostatic discharge cycling experiment between0.01 V and 1.6 V versus Li metal; ³Capacity calculated from the 2^(nd)cycle discharge in a C/20 galvanostatic cycling experiment between 0.01V and 1.6 V versus Li metal; ⁴Capacity calculated from 2^(nd) dischargein a C/3 galvanostatic cycling experiment between 4.4 V and 2 V.

Example 9 Static Cell Tests of Full Lithium Ion Cell Using Cathode andAnode Semi-Solid Suspensions

Cathode and anode slurries, as described in Examples 6 and 7, werecharged and discharged electrochemically against each other in a static,electrochemical cell. The cathode and anode slurries were each placed inmetallic wells/current collectors of the dimensions described in Example8. The wells/current collectors were made of aluminum and copper for thecathode and anode, respectively. A Celgard 2500 film separated the twoslurries in the cell. The cathode and anode suspensions were charged anddischarged relative to each other repeatedly under potentiostatic andgalvanostatic conditions, with galvanostatic testing being done atC-rates ranging from C/20 to C/10. A representative plot of voltage as afunction of time is shown in the lower panel in FIG. 8. Thecorresponding charge or discharge capacity is shown in the upper panelin FIG. 8. In this test, the cell was charged under potentiostaticconditions, holding the cell voltage at 4.4V, while the charge capacitywas monitored. The rate of charging is initially high, then diminishes.The cell was then galvanostatically discharged at a C/20 rate. Thecapacity obtained in the first discharge is ˜3.4 mAh, which is 88% ofthe theoretical capacity of the anode in the cell. There is an excess ofcathode in this cell which is therefore not fully utilized.

Example 10 Lithium Titanate Spinel Anode Suspension

Lithium titanate spinel, which may have a range of Li:Ti:O ratios andalso may be doped with various metals or nonmetals, and of which anon-limiting composition is Li₄Ti₅O₂, intercalates lithium readily at athermodynamic voltage near 1.5V with respect to Li/Li⁺, and increases inits electronic conductivity as Li is inserted due to the reduction ofTi⁴⁺ to Ti³⁺. A 5 g sample of lithium titanate spinel powder is mixedwith 100 mg of Ketjen Black and suspended in 10 mL of a standard lithiumion battery electrolyte, and the suspension is sonicated for 20 minutes.Such a suspension does not separate into components for at least 48hours. This suspension was charged and discharged in a lithium half-cellas described in Example 8. FIG. 10 shows the galvanostatic lithiuminsertion and extraction curves for the suspension at a relatively highC/1.4 rate. During the lithium insertion step, the average voltage isvery near the thermodynamic voltage of 1.55V, while upon extraction theaverage voltage is somewhat higher.

Example 11 Flowing Half Cell Tests on Cathode and Anode Slurries

Samples, as described in Examples 6 and 7, were charged and dischargedelectrochemically against a lithium metal electrode in a flowing,electrochemical cell. The cathode or anode slurry was pumped into ametallic channel of defined geometry, which acted as the currentcollector. The current collectors were aluminum and copper for thecathode and anode, respectively. Channels were 5 mm in diameter, 50 mmin length, and had a depth of 500 μm. A porous PVDF sheet (pore size:250 μm), sandwiched between 2 Celgard 2500 separator films, addedmechanical strength. In between the two separator films, separated fromthe slurries, was a lithium metal reference electrode attached to acopper wire and electrically isolated from both current collectors. Anexcess of liquid electrolyte was added to the gaps in the device toensure that the electrochemically active components remained immersed inliquid electrolyte. Testing was conducted in an argon-filled glove box.The slurry in the channel was charged and discharged at rates rangingfrom C/20 to C/5. During charging, uncharged slurry was mechanicallypumped into the test cell to replace that which had been fully chargedin the channel. The charged slurry was pumped out of the cell and storeduntil the end of the charge. For discharging, the cell was run inreverse, both electrochemically and mechanically. New volume of slurrywas pumped into the test cell as the volume in the cell was fullydischarged. The volume of discharged suspension was pumped out of thecell and stored until the end of the discharge.

Example 12 Flowing Full Cell Tests on Cathode and Anode Slurries

Cathode and anode slurries, as described in Examples 3 and 4, werecharged and discharged electrochemically in concert in a flowing,electrochemical cell. The cathode or anode slurry was pumped into ametallic channel, the channel material also acting as the currentcollector. The current collectors were aluminum and copper for thecathode and anode, respectively. Channels were 5 mm in diameter, 50 mmin length, and had a depth of 500 μm. A 250 μm perforated PVDF sheet,sandwich between 2 Celgard 2500 films, added mechanical strength andseparated one slurry channel from the other. A piece of lithium foilattached to a copper wire was also sandwiched between the separatorfilms and acted as a reference electrode. The slurries in the channelwere charged and discharged at rates ranging from C/20 to C/5. Usingperistaltic pumps, to which were attached elastomer tubing filled withcathode and anode slurries feeding the respective channels in theelectrochemical cells, the slurries were pumped through the channels.During charging, uncharged slurry was mechanically pumped into the testcell to replace that which was fully charged. For discharging, the cellwas run in reverse, both electrochemically and mechanically. The twoslurries were flowed independent of one another and the state of chargeof both anode and cathode slurries were monitored in real time using thelithium metal reference electrode. Several different modes of operationwere used. In one instance, one or both slurries were intermittentlypumped into the channels, the pumping stopped, and the slurries in thechannel were charged or discharged, following which the slurry in thechannel was displaced by fresh slurry and the process repeated. Inanother mode of operation, the slurries were pumped continuously, withthe residence time of each slurry in its respective channel beingsufficient for complete charge or discharge before exiting the channel.In yet another mode of operation, one or both slurries were pumpedthrough their respective channels at a rate too high for completecharging or discharging during the residence time, but the slurry wascontinuously circulated so that over time, all of the slurry in thesystem was either charged or discharged. In yet another mode ofoperation, the pumping direction of one or both slurries wasperiodically reversed during a charging or discharging step, causingmore slurry than the channel can accommodate at a given time to becharged or discharged.

Example 13 In-Line Electrochemical Sensor

FIG. 11 includes a schematic illustration of a design for an in-lineelectrochemical sensor in which a flow channel allows the positiveand/or negative flowable redox composition to flow past a galvanostaticsensor. The sensing elements include a lithium metal electrode connectedto one terminal of the sensor, and a metal electrode connected to theother terminal of the sensor. The lithium metal electrode can beelectronically isolated from the positive or negative flowable redoxcomposition by a an ionically conducting separator layer, which may be asolid inorganic or organic ionic conductor, or a porous separator filminfused with liquid electrolyte. In this particular example theelectronically isolated metal electrode is a layer of microporouslithium ion battery polymer separator film. FIG. 12 includes a plot ofpotential as a function of time measured for a flowing semisolidsuspension formulated with 20 vol % of a lithium titanate spinel activematerial and 5 vol % of Ketjen black in a nonaqueous electrolytecomprising 1M LiPF₆ in a mixture of alkyl carbonates. This plotdemonstrates that the open circuit voltage of the suspension can bemonitored over time, showing in this instance that the lithium titanatespinel is in a highly delithiated condition, since upon lithiuminsertion the open circuit voltage of this material is 1.55V withrespect to Li/Li⁺.

Example 14 Electronically Conductive Semi-Solid Suspensions

The rheology and electronic conductivity of semi-solid suspensionscontaining a small percentage of a high surface area nanoparticulateconductive carbon was measured, and showed that electronicallyconductive suspensions that are analogous to “liquid wires” areproduced. In this example, the active materials tested are lithiumcobalt oxide (LiCoO₂, from AGC Seimi Chemical Co. Ltd, Kanagawa,Japan)), lithium titanate spinel (Li₄Ti₅O₁₂, from AltairNano, Reno,Nev.), and Ketjen black (ECP600JD, Akzo Nobel Polymer Chemicals LLC,Chicago, Ill.). Prior to use, the LiCoO₂ was jet-milled to reduce itsparticle size, and the Li₄Ti₅O₁₂ (LTO) was heated under a gas mixture ofAr and H₂ in a 95:5 ratio at 800° C. for 20 hours in a quartz tubeplaced inside a Lindberg/Blue M furnace, in order to reduce the oxideand increase its electronic conductivity. After heat treatment, thecolor of the powder had changed from white to blue. The active materialor carbon or both were weighed and mixed in a 20 mL glass vial and thesolid mixture was suspended by addition of a conventional lithium-ionbattery electrolyte using LiPF₆ as the lithium salt in a mixture ofalkyl carbonates (Novolyte Technologies, Independence, Ohio). Theresulting suspension was mixed and sonicated in a Branson 1510ultrasonic bath for between 20 and 60 minutes. The viscosities of theparticle suspensions in electrolyte were measured inside an argon-filledglove box using a Brookfield Digital Viscometer, mode DV-II+Pro Extra.The electrical conductivity of suspensions with and without flow wasmeasured in an apparatus with a flow channel on either side of whichwere stainless steel electrodes, and an insulating body made of PVdFpolymer. From the channel diameter and flow velocity, the shear rate ofthe fluid can be determined.

The measurements showed that the dispersed, electronically conductivecarbon forms physically and electronically percolating networks withinthe flowable suspensions that enable charge and discharge of thesemisolid suspensions. FIG. 13A shows viscosity versus shear rate forsuspensions of nanoparticulate carbon (Ketjen black) and LiCoO₂ (LCO) inalkyl carbonate electrolyte. The suspensions show shear thinningbehavior consistent with the presence of Ketjen networks that arepartially disrupted by shear stress. Thus, even at dilute concentrationsof the carbon additive (in the case, <1% volume fraction of Ketjenblack), both of the suspensions containing LiCoO₂ (LCO) particles andKetjen exhibit strong shear-thinning behavior characteristic of thecarbon additive. FIG. 13B shows a Nyquist plot that shows the ionic andelectronic conductivity of the different suspensions and theircomponents. The ionic conductivity of suspensions was set by theelectrolyte, while the electronic conductivity was set by the Ketjenblack network. For 0.6% Ketjen black, the conductivity under non-flowingconditions was 1.2 mS/cm, while that under a high shear rate of 257sec⁻¹ was reduced only slightly to 0.67 mS/cm, showing that electronicconductivity was maintained. Note that 0.6% Ketjen imparted similarelectronic conductivity to suspensions containing either LCO or LTO. Theconductivity of the LCO and Ketjen mixture was 0.06 mS/cm, and theconductivity of the (Li₄Ti₅O₁₂) LTO and Ketjen suspension was 0.01mS/cm. The results show that while shear (due to flow) can change theelectrical conductivity of the suspension, indicative of carbon networksthat are physically altered by shear, it is possible to producesuspension formulations that have very low fractions of carbon additivesand yet which have high electronic conductivities comparable to theionic conductivities of the liquid electrolyte. Such a situation isdesirable given that the semi-solid electrode materials should, likeother battery electrodes, exhibit mixed electronic-ionic conductivity.

Example 15 Nonaqueous Flow Cell Using Cathode Semi-Solid Suspension

A semi-solid suspension having by mass 51.3% LiCoO₂, 0.7% Ketjen black,and 48% of a nonaqueous electrolyte consisting of LiPF₆ in a mixture ofalkyl carbonates (by volume: 22.4% LiCoO₂, 0.7% Ketjen black, and 76.9%electrolyte) was prepared according to the method of Example 14. FIG. 14shows a schematic of the electrochemical cell configuration. Thenegative half was machined from 101 copper alloy and the positive halffrom 6061 aluminum alloy. The flow channel has a 1/16″ diameter and avolume of 0.16 ml. A stationary lithium metal foil electrode was affixedto the channel in the copper negative half of the cell, and separatedfrom the flowing cathode semi-solid by a layer of microporous separator(Tonen Chemical Corporation, Japan). The working surface of the 6061aluminum alloy cell components was sputtered with gold to reduceinterfacial impedance. Electrochemical testing was performed using aSolartron potentiostat operating a 1400 Cell Test System (AMETEK Inc.,Paioli, Pa., USA). Continuous flow experiments were performed using aMasterflex peristaltic pump (Masterflex, Vernon Hills, Ill., USA);Chem-Sure™ tubing (W.L. Gore and Associates, Elktron, Md., USA) was usedinside the pump and was connected to the cell using MasterflexChem-Durance™ tubing. The semi-solid was continuously circulated at 20.3mL/min through the single channel half-flow-cell depicted in FIG. 14,while conducting multi-step galvanostatic charging/discharging between 2and 4.4V (rate varied between C/8.8 and D/4.4). FIG. 15 includes plotsof the state of charge, current, and voltage as a function of time forthe cycled cell. The charge capacity at 4.4V (rest voltage 4.2V)corresponds to a LiCoO₂ specific capacity of 146 mAh/g (system-value),while the discharge capacity corresponds to 127 mAh/g. Compared to theexpected reversible capacity of about 140 mAh/g, these valuesdemonstrate high utilization of the system's LiCoO₂. Note that the 12.5%lower discharge capacity was obtained at a higher average discharge thancharge rate and does not represent the maximum achievable coulombicefficiency.

Example 16 Nonaqueous Flow Cell Using Anode Semi-Solid Suspension

A semi-solid suspension was prepared, having by mass 13.1% Li₄Ti₅O₁₂(heat treated in reducing ambient as in Example 14), 1.7% Ketjen black,and 85.1% of a nonaqueous electrolyte consisting of 70:30 mass ratio of1,3-dioxolane and LiBETI salt (by volume: 5.8% Li₄Ti₅O₁₂, 1.2% Ketjenblack, and 93% electrolyte). The suspension was flowed through a celllike that in Example 15, with a 1/16″ diameter channel, at a flow rateof 10 mL/min. A lithium metal foil counterelectrode was used, as inExample 15. However, instead of being charged galvanostatically, thecell was charged potentiostatically, first at 1.35V and then at 1.0V, asshown in FIG. 16. After being fully charged potentiostatically, the cellwas discharged galvanostatically at a current density of 17.1 A/m², thearea being that of the microporous separator (made by Tonen). FIG. 16shows the voltage, charge capacity, and current as a function of time.It is seen that the continuously flowing semi-solid suspension can becharged with a capacity of about 180 mAh/g and discharged with adischarge capacity of about 140 mAh/g, the specific capacity being thatfor the Li₄Ti₅O₁₂ alone, indicating substantially complete charging anddischarging of the anode compound.

Example 17 Dual Electrolyte Lithium Ion Cell

The stability of common solvents found in commercial Li-ion batteries isdirectly dependent on the most polar chemical group found in themolecule. The most stable molecules are, going from 5 to 0 V vs. aLi/Li⁺ electrode: carbonates (such as dimethyl carbonate), esters (suchas γ-butyrolactone) and ethers (such as 1,2-dimethoxyethane,tetrahydrofurane or 1,3-dioxolane). At the low insertion potentials ofmany anodes, solvent reduction can form detrimental insulatingsolid-electrolyte interface (SEI) films. This example demonstrates theuse of dual fluid electrolytes in a redox flow energy storage device. Asemi-solid anode suspension was prepared, having by mass 34.0% Li₄Ti₅O₁₂(reduced as in Example 14), 1.1% Ketjen black, and 64.9% of a nonaqueouselectrolyte consisting of 70:30 mass ratio of 1,3-dioxolane and LiBETIsalt (by volume: 17.0% Li₄Ti₅O₁₂, 0.9% Ketjen black, and 82.1%electrolyte). The two suspensions were separated by a layer ofmicroporous separator in a non-flowing cell. FIG. 17 shows galvanostaticcharge-discharge curves for the cell, measured at a current density of9.4 A/m² of separator. The charge and discharge voltages are as expectedfor this electrochemical couple, taking into account cell polarization.Since the cell has an excess of cathode capacity to anode capacity, thecell specific capacity is calculated with respect to the mass ofLi₄Ti₅O₁₂ anode, and is about 130 mAh/g, showing good utilization of theactive material in this semi-solid lithium ion cell.

Example 18 Ether Based Electrolyte for Semi-Solid Electrodes

The stability of common solvents found in commercial Li-ion batteries isdirectly dependent on the most polar chemical group found in themolecule. The most stable molecules are, going from 5 to 0 V vs. aLi/Li⁺ electrode: carbonates (such as dimethyl carbonate), esters (suchas γ-butyrolactone) and ethers (such as 1,2-dimethoxyethane,tetrahydrofurane or 1,3-dioxolane). At the low insertion potentials ofmany anodes, solvent reduction can form detrimental insulatingsolid-electrolyte interface (SEI) films. This example demonstrates theuse of ether based electrolytes to provide electrochemical stabilityunder such conditions. A semi-solid anode suspension was prepared,having by mass 51.8% MCMB graphite (grade 6-28, Osaka Gas Co., Osaka,Japan) and 48.2% of an electrolyte consisting of 2M LiClO₄ in1,3-dioxolane. By volume, the semi-solid has 40% MCMB and 60%electrolyte. The semi-solid anode was tested vs. a lithium metal anodein an non-flowing cell under C/8 galvanostatic conditions (currentdensity 18.7 A/m² of Tonen separator). FIG. 18 shows the voltage vs.capacity results, which show that despite the low insertion potential,and absence of conductive additives, reversible cycling is obtained.

Example 19 Ionic Liquid Electrolyte for Semi-Solid Electrod

In linear ethers with 4 or more oxygen atoms, the solvent can wraparound the Li⁺ ion to form a more stable coordinated cation. At 1:1molar ratios of solvent to salt, the product of mixing is an ionicliquid of the formula [Li(ether)](anion). Such ionic liquids have beenproven to be electrochemically stable in the 0 to 5 V potential rangevs. the Li/Li⁺ electrode, which makes them suitable electrolytes for 4 Vsemi-solid flow cells. FIG. 19 shows the voltage vs. capacity curve forC/11 galvanostatic cycling (3.4 A/cm² of Celgard 2500 separator, CelgardLLC, Charlotte, N.C.) of a cathode semi-solid suspension having by mass28.1% LiCoO₂, 3.4% Ketjen black, and 68.5% of tetraglyme and lithiumbis(trifluoromethane)sulfonimide in a 1:1 molar ratio (referred to as[Li(G4)]TFSI), tested against a lithium metal counterelectrode in anon-flowing cell. By volume, the semi-solid has 10.0% cathode, 3.0%Ketjen, and 87.9% electrolyte. The cathode specific capacity withrespect to the LiCoO₂ alone is about 120 mAh/g, showing good utilizationof the semi-solid suspension.

Example 20 Cathode-Anode-Electrolyte Combinations for Semi-Solid FlowCells

Selection of a suitable cathode-anode-electrolyte depends on thepotentials at which the cathode and anode store ions, as well as thestability window of the electrolyte. Table 3 shows several suitablecombinations. SSDE refers to LiPF₆ in a mixture of alkyl carbonates; DMCrefers to LiPF₆ in dimethyl carbonate; DXL refers to 2M LiClO₄ in1,3-dioxolane; DOL refers to 70:30 mass ratio of 1,3-dioxolane andLiBETI; and Li(G4)]TFSI refers to tetraglyme and lithiumbis(trifluoromethane)sulfonimide in a 1:1 molar ratio. For example:

Olivine cathodes such as lithium iron phosphate or lithium manganesephosphate or their solid solutions, or doped and nanoscale olivines, canbe used with Li₄Ti₅O₁₂ (LTO) in DMC based electrolytes. Such systemswill have an operating voltage of 1.9 V to 2.5V. Power and cycle life isexpected to be excellent for nanoscale active materials in thesesystems.

LiMn₂O₄—graphite used with DXL has a higher cell voltage of 2.8 V.

Li₂MnO₃.LiMO₂—LTO used with DMC has a cell voltage of 2.9 V. This highcapacity cathode when used with the higher voltage LTO anode still has ahigh cell voltage and is expected to have high anode stability.

Li₂MnO₃.LiMO₂ when used with a high capacity anode such as that producedby 3M, or Si, or even graphite, and used with [Li(G4)]TFSI electrolytewill have a high energy density due to the high capacity of both cathodeand anode, and the higher cell voltage: of 3.9-4.3 V. Note that thecycle life of high capacity anodes such as Si and the 3M alloy, whichundergo large volume changes as they are charged and discharged, islikely to be improved in our semi-solid electrodes since the activematerials particles are free to expand and contract within the liquidphase without generating large stresses as they do in a conventionalelectrode.

TABLE 3 Comparison of voltage (vs. Li/Li⁺) of several Li-ion cathode andanode materials and the stability ranges of electrolytes, showingsystems suitable for semi-solid flow cells.

*M = Co, Ni, Mn.

Example 21 Aqueous Lithium Ion Semi-Solid Flow Battery UsingLiV₃O₈—LiCoO₂ in 5 M LiNO₃ Electrolyte

This example is illustrative of a semi-solid flow battery that useslithium metal oxides as the cathode and anode in conjunction with anaqueous lithium-ion conducting electrolyte.

Preparation of Aqueous Lithium Vanadium Oxide Anode Slurry:

Lithium vanadium oxide powder with a composition of LiV₃O₈ issynthesized via a solid state reaction method. LiOH is mixed with V₂O₅in amounts producing the stoichiometric ratio of Li to V in LiV₃O₈. Thepowder mixture is ball milled for 24 hrs in methanol and then dried at60° C. The dried powder mixture is then fired at 600° C. for 24 hr inair resulting in the LiV₃O₈ compound. To prepare the anode slurry,15-30% by volume of lithium vanadate is mixed with 0.5-2% by volume ofCOOH functionalized carbon nanotube as a conductive additive. The drypowders are first mixed using a vortex mixer for 2 mins. The balance ofthe suspension is an electrolyte consisting of a 5 M solution of LiNO₃in water. This electrolyte is added to the powder mixture and themixture is sonicated for 1 hr to obtain the anode slurry.

Preparation of Aqueous Lithium Cobalt Oxide Cathode Slurry:

Suspensions containing 35% by volume of lithium cobalt oxide, 0.6-2% byvolume of COOH functionalized carbon nanotubes, and the balance being anelectrolyte consisting of a 5M solution of LiNO₃ in water, are prepared.For example, 3.5 g of lithium cobalt oxide is mixed with 0.0259 g of thecarbon using a vortex mixture for 2 mins. Afterwards, the electrolyte isadded in the appropriate amount to make up the balance of the semi-solidsuspension, and mixture is sonicated for 1 hr.

The cathode and anode suspensions are used in a flow cell of the designin Example 15, but in which both current collectors are fabricated froma high chromium content stainless steel. Optionally, the stainless steelin contact with the semi-solid suspensions is coated with gold byelectroplating. The separator membrane is selected to be one withimproved wetting by an aqueous electrolyte, such as Celgard 3501.

Example 22 Lithium Ion Semi-Solid Flow Battery Using LiFePO₄—Li₄Ti₅O₁₂in Aqueous Electrolyte Having 1 M LiNO₃ and 1M LiOH

This example is illustrative of a semi-solid flow battery that useslithium metal oxides as the cathode and anode in conjunction with anaqueous lithium-ion conducting electrolyte, and has a relatively highoperating voltage.

Preparation of an Aqueous Lithium Titanate Spinel Anode Slurry:

A suspension containing 30% by volume of lithium titanium oxide(Li₄Ti₅O₁₂) and 1% by volume of Ketjen black as the conductive additivein 69% by volume of an aqueous electrolyte containing 1M each of LiNO₃and LiOH is prepared by first mixing 2.076 g Li₄Ti₅O₁₂ and 0.0432 g ofthe Ketjen black in the dry state using a vortex mixer for 2 mins. 1.38ml of the electrolyte is then added and the mixture is sonicated for 1hr.

Preparation of an Aqueous Lithium Iron Phosphate Cathode Slurry:

A suspension containing 20% by volume of a carbon-coated lithium ironphosphate with 1% by volume of Ketjen black in 69% by volume of anaqueous electrolyte containing 1M each of LiNO₃ and LiOH is synthesizedby mixing 3.577 g of lithium iron phosphate and 0.108 g of carbon in thedry state for 2 min using a vortex mixer. 3.95 ml of the electrolyte isthen added, and the mixture is sonicated for 1 hr.

The cathode and anode suspensions are used in a flow cell of the designin Example 15, but in which both current collectors are fabricated froma high chromium content stainless steel. Optionally, the stainless steelin contact with the semi-solid suspensions is coated with gold byelectroplating. The separator membrane is selected to be one withimproved wetting by an aqueous electrolyte, such as Celgard 3501.

Example 23 Aqueous Sodium Ion Semi-Solid Flow Battery UsingNa_(x)MnO₂—Activated Carbon in 1 M Na₂SO₄ Electrolyte

This example is illustrative of an aqueous sodium ion semi-solid flowbattery that uses a sodium metal oxide as the cathode and activatedcarbon as the anode. The cathode stores Na primarily by an intercalationreaction, while the anode stores Na primarily by surface adsorption.

Preparing an Aqueous Activated Carbon Anode Slurry:

A suspension containing 20% by volume of activated carbon (Darco, G-60)and 80% by volume of a 5M LiNO₃ aqueous electrolyte is prepared. 0.65 gof activated carbon is mixed with 1.2 ml of an electrolyte consisting of1M Na₂SO₄ in water. The mixture is sonicated for 1 hr.

Preparing an Aqueous Sodium Manganese Oxide Cathode Slurry:

Sodium manganese oxide powder with a composition of Na_(0.44)MnO₂ issynthesized via a solid state reaction method. NaCO₃ powder is mixedwith MnCO₃ powder in a ratio producing the Na:Mn stoichiometry of theoxide compound. The powder mixture is ball milled in methanol for 24 hrsand then dried at 60° C. The homogeneously mixed powder is then fired at300° C. for 8 hr in air. After firing at 300° C. the powder is groundand is re-fired at 800° C. for another 9 hrs in air to obtainNa_(0.44)MnO₂. A cathode suspension containing 20% by volume of thesodium manganese oxide and 2% by volume of COOH functionalized carbonnanotubes as the conductive additive is mixed with 78% by volume of anelectrolyte consisting of a 5M solution of LiNO₃ in water. Specifically,1.269 g of sodium manganese oxide is mixed with 0.0648 g of the COOHfunctionalized carbon nanotubes. The dry powders are mixed using avortex mixer for 2 mins. Afterwards, 1.17 ml of the 1 M Na₂SO₄electrolyte is added to the powder mixture and the mixture is sonicatedfor 1 hr.

The cathode and anode suspensions are used in a flow cell of the designin Example 15, but in which both current collectors are fabricated froma high chromium content stainless steel. Optionally, the stainless steelin contact with the semi-solid suspensions is coated with gold byelectroplating. The separator membrane is selected to be one withimproved wetting by an aqueous electrolyte, such as Celgard 3501.

It is recognized, of course, that those skilled in the art may makevarious modifications and additions to the processes of the inventionwithout departing from the spirit and scope of the present contributionto the art. Accordingly, it is to be understood that the protectionsought to be afforded hereby should be deemed to extend to the subjectmatter of the claims and all equivalents thereof fairly within the scopeof the invention.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Other elements may optionallybe present other than the elements specifically identified by the“and/or” clause, whether related or unrelated to those elementsspecifically identified unless clearly indicated to the contrary. Thus,as a non-limiting example, a reference to “A and/or B,” when used inconjunction with open-ended language such as “comprising” can refer, inone embodiment, to A without B (optionally including elements other thanB); in another embodiment, to B without A (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of,” or“exactly one of.” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” and the like are to be understoodto be open-ended, i.e., to mean including but not limited to. Only thetransitional phrases “consisting of” and “consisting essentially of”shall be closed or semi-closed transitional phrases, respectively, asset forth in the United States Patent Office Manual of Patent ExaminingProcedures, Section 2111.03.

What is claimed is:
 1. An energy storage device, comprising: a positiveelectrode current collector, a negative electrode current collector, andan ion-permeable membrane separating the positive current collector andthe negative current collector; a positive electrode disposed betweenthe positive electrode current collector and the ion-permeable membrane;the positive electrode current collector and the ion-permeable membranedefining a positive electroactive zone accommodating the positiveelectrode; and a negative electrode disposed between the negativeelectrode current collector and the ion-permeable membrane; the negativeelectrode current collector and the ion-permeable membrane defining anegative electroactive zone accommodating the negative electrode;wherein at least one of the positive electrode and the negativeelectrode includes a semi-solid or condensed liquid ion-storing redoxcomposition, the semi-solid or condensed liquid ion-storing redoxcomposition including a conductive additive selected from metalcarbides, metal nitrides, carbon black, graphitic carbon powder, carbonfibers, carbon microfibers, vapor-grown carbon fibers (VGCF),fullerenes, carbon nanotubes (CNTs), multiwall carbon nanotubes (MWNTs),single wall carbon nanotubes (SWNTs), graphene sheets, and materialscomprising fullerenic fragments that are not predominantly a closedshell or tube of the graphene sheet, and mixtures thereof, and whereinthe semi-solid or condensed liquid ion-storing redox composition iscapable of taking up or releasing ions, remains substantially insolubleduring operation of the cell, and has a thickness of about 250 μm toabout 800 μm.
 2. The energy storage device of claim 1, wherein thesemi-solid or condensed liquid ion-storing redox composition forms acontinuously electronically conductive network percolative pathway tothe negative current collector and/or the positive current collector. 3.The energy storage device of claim 1, wherein the positive electrode andthe negative electrode include a semi-solid or condensed liquidion-storing redox composition.
 4. The energy storage device of claim 1,wherein one of the positive electrode and the negative electrodeincludes a semi-solid or condensed liquid ion-storing redox compositionand the other electrode is a solid electrode.
 5. The energy storagedevice of claim 1, wherein the ion storage compound stores at least oneof Li, Na or H.
 6. The energy storage device of claim 1, wherein theconductive additive forms a percolative conductive continuouslyelectronically conductive network in the semi-solid or condensed liquidion-storing redox composition.
 7. The energy storage device of claim 1,wherein the semi-solid or condensed liquid ion-storing redox compositionis free of added binder.
 8. An energy storage device, comprising: apositive electrode current collector, a negative electrode currentcollector, and an ion-permeable membrane separating the positive currentcollector and the negative current collector; a positive electrodedisposed between the positive electrode current collector and theion-permeable membrane; the positive electrode current collector and theion-permeable membrane defining a positive electroactive zoneaccommodating the positive electrode; and a negative electrode disposedbetween the negative electrode current collector and the ion-permeablemembrane; the negative electrode current collector and the ion-permeablemembrane defining a negative electroactive zone accommodating thenegative electrode; wherein at least one of the positive electrode andthe negative electrode includes a semi-solid or condensed liquidion-storing redox composition, the semi-solid or condensed liquidion-storing redox composition including a conductive additive, whereinthe volume percentage of the ion-storing solid phase is between 5% and70%, and the volume percentage of the total solids including theconductive additive is between 10% and 75%, and wherein the semi-solidor condensed liquid ion-storing redox composition is capable of takingup or releasing ions, remains substantially insoluble during operationof the cell, and has a thickness of about 250 μm to about 800 μm.
 9. Theenergy storage device of claim 8, wherein the semi-solid or condensedliquid ion-storing redox composition forms a continuously electronicallyconductive network percolative pathway to the negative current collectorand/or the positive current collector.
 10. The energy storage device ofclaim 8, wherein the positive electrode and the negative electrodeinclude a semi-solid or condensed liquid ion-storing redox composition.11. The energy storage device of claim 8, wherein one of the positiveelectrode and the negative electrode includes a semi-solid or condensedliquid ion-storing redox composition and the other electrode is a solidelectrode.
 12. The energy storage device of claim 8, wherein the ionstorage compound stores at least one of Li, Na or H.
 13. The energystorage device of claim 8, wherein the conductive additive forms apercolative conductive continuously electronically conductive network inthe semi-solid or condensed liquid ion-storing redox composition. 14.The energy storage device of claim 8, wherein the semi-solid orcondensed liquid ion-storing redox composition is free of added binder.